Simulated Interaction Between 

Freshwater and Saltwater and Effects of 

Ground-Water Pumping and Sea-Level 

Change, Lower Cape Cod Aquifer System, 

Massachusetts

By John P. Masterson

In cooperation with the 

National Park Service, 

Massachusetts Executive Office of Environmental Affairs, 

Cape Cod Commission, and the 

Towns of Eastham, Provincetown, Truro, and Wellfleet

Scientific Investigations Report 2004-5014

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to

reproduce any copyrighted materials contained within this report.

Masterson, J.P., 2004, Simulated interaction between freshwater and saltwater and effects of ground-water pumping

and sea-level change, Lower Cape Cod aquifer system, Massachusetts: U.S. Geological Survey Scientific Investigations

Report 2004-5014, 72 p.
iii

CONTENTS

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Depositional History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Geologic Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Hydrologic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Simulation of Ground-Water Flow in the Lower Cape Cod Aquifer System. . . . . . . . . . . . . . . . . . . . . . . . . 8

Ground-Water Recharge Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Altitude and Configuration of Water-Table Mounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Interaction Between Ground and Surface Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Controls of Hydrogeologic Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Simulated Interaction Between Freshwater- and Saltwater-Flow Systems . . . . . . . . . . . . . . . . . . . . . . . 12

Effects of Surface-Water Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Effects of Ground-Water Pumping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Effects of Sea-Level Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Water Levels and Streamflows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Freshwater/Saltwater Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Pumping Wells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Simulation of Proposed Ground-Water-Pumping Scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Effects on Streamflow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Effects on Water Levels in Kettle-Hole Ponds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Effects on the Movement of the Freshwater/Saltwater Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Simulated Effects of Local Sea-Level Change Through Removal of a Tide-Control Structure . . . . . . . 41

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Appendix: Development of Ground-Water Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figures

1–3. Maps showing:

1. Location of the four flow lenses of the Lower Cape Cod aquifer system and

model-calculated water-table contours, Cape Cod, Massachusetts. . . . . . . . . . . . . . . . . .3

2. Ice recession and lobe formation in southeastern Massachusetts . . . . . . . . . . . . . . . . . . .4

3. Surficial geology of Lower Cape Cod and the depositional sequence of the 

Wellfleet, Truro, and Eastham outwash plains. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
iv

4–6. Diagrams showing:

4. Deltaic deposits prograding into a glacial lake, including topset, foreset, 

and bottomset deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5. The Lower Cape Cod aquifer system, Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6. Area contributing recharge to a pumping well in a simplified, hypothetical 

ground-water-flow system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7, 8. Maps showing: 

7. The delineation of ground-water-recharge areas to public-supply wells, 

ponds, streams, and coastal areas for current (2002) average pumping and 

recharge conditions, Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Model-calculated delineation of the boundary between freshwater and 

saltwater beneath the Lower Cape Cod aquifer system, Cape Cod . . . . . . . . . . . . . . . . . 13

9. Model section A-A′ showing the model-calculated boundary between 

freshwater and saltwater flow, Lower Cape Cod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Model section B-B′ showing the model-calculated boundary between 

freshwater and saltwater flow, Lower Cape Cod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

11. Diagram of the Lower Cape Cod aquifer system showing lateral and vertical 

saltwater intrusion in response to ground-water pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

12. Map showing locations of existing (2002) and proposed public-supply wells and 

Traffic Analysis Zones, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

13. Profiles of natural gamma and electromagnetic (EM) geophysical logs at the 

Knowles Crossing well field, North Truro, measured in September 2000 . . . . . . . . . . . . . . . . . 18

14. Diagram showing lateral and vertical saltwater intrusion beneath the Knowles 

Crossing well field, North Truro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

15, 16. Graphs showing:

15. Specific conductance in monitoring wells TSW-259 and TSW-260 beneath 

Knowles Crossing well number 2 (KC-2) and total pumping in 2001 at the 

Knowles Crossing well field, North Truro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

16. Model-calculated freshwater/saltwater interface and simulated pumping 

from 1955–2050 at the South Hollow well field, North Truro . . . . . . . . . . . . . . . . . . . . . . . . 21

17. Profiles of electromagnetic (EM) logs measured in September 2000 and modelcalculated changes in salt concentration for current (2002) conditions at the 

South Hollow well field, North Truro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

18. Graph showing water-table altitude at observation well TSW-1, North Truro, 

1950–2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

19. Map showing locations of long-term observation wells and the measured and 

model-calculated increase in the altitude of the water table with time, Lower 

Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

20, 21. Graphs showing:

20. Model-calculated water-table altitude from 1929 to 2050 at Sites X and Y and 

simulated changes in sea level A, above NGVD 29; and B, above local sea 

level, North Truro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

21. Model-calculated altitude from 1929 to 2050 of the freshwater/saltwater 

interface relative to NGVD 29 beneath Sites X and Y, North Truro . . . . . . . . . . . . . . . . . . 27

22. Diagram showing a hypothetical aquifer showing ground-water discharge to a 

surface-water body with A, no pumping; B, pumping at a rate such that the well 

would capture water that would otherwise discharge to the surface-water body; 

and C, pumping at a higher rate so that the flow direction is reversed and the well 

pumps water from the surface-water body. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
v

23–25. Maps showing:

23. Location of model-calculated contributing areas to A, Hatches Creek for 

current (2002) conditions; B, Hatches Creek and the Roach site pumping at 

0.55 million gallons per day; C, Hatches Creek and Water District G site pumping 

at 0.55 million gallons per day; and D, Hatches Creek, Water District G site, and

the Roach site each pumping at 0.55 million gallons per day, Eastham. . . . . . . . . . . . . . .32

24. Location of model-calculated contributing areas to Hatches Creek and the 

proposed Water District G well pumping at 0.55 and 1.10 million gallons per day, 

Eastham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

25. Location of A, model-calculated contributing area to Duck Pond and 

water-table contours for current (2002) conditions; and B, model-calculated 

contributing areas to Duck Pond, Coles Neck well, Boy Scout Camp site, and the 

Wellfleet By The Sea site, each pumping at 0.10 million gallons per day, and 

changes in model-calculated water levels from current (2002) conditions, 

Wellfleet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

26. Graph showing model-calculated monthly pond-level altitudes in Duck Pond for 

current (2002) conditions and simulated pumping conditions of 0.10 million gallons 

per day at the Coles Neck well, the Boy Scout Camp site, and the Wellfleet By 

the Sea site, Wellfleet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

27–29. Maps showing:

27. Model-calculated water-table contours and contributing areas to A, South 

Hollow well field and the North Truro Air Force Base wells 4 and 5 for current 

(2002) pumping rates; B, South Hollow well field pumping at 0.80 million gallons 

per day and North Truro Air Force Base wells 4 and 5 pumping at current (2002) 

rates; and C, South Hollow well field and the North Truro Air Force Base wells 4 

and 5 for current (2002) pumping rates and North Unionfield site pumping at 

0.80 million gallons per day, North Truro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

28. Model-calculated water-table contours and contributing areas to A, Little 

Pamet River, South Hollow well field, and North Truro Air Force Base 

wells 4 and 5 for current (2002) pumping rates; and B, Little Pamet River, 

South Hollow well field, and North Truro Air Force Base wells 4 and 5 for 

current (2002) pumping rates and CCC-5 site pumping at 0.8 million gallons per 

day, Truro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

29. Model-simulated boundary conditions with A, the existing Herring River 

tide-control structure and conditions in the Chequesset Neck area; and 

B, proposed tide-control structure and the variable saltwater concentrations 

used in simulations 1 and 4, Wellfleet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

Tables

1. Model-calculated hydrologic budget for the four flow lenses of the Lower Cape Cod 

aquifer system under current (2002) pumping and recharge conditions, Cape Cod, 

Massachusetts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

 2. Model-calculated changes in the altitude of the freshwater/saltwater interface in 

the vicinity of the Herring River tide-control structure, Wellfleet, in response to 

changes in simulated salt concentrations and stream stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
vi

CONVERSION FACTORS, DATUMS, AND

ABBREVIATIONS

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29). Horizontal coordinate information is referenced to the North American Datum of 1927 

(NAD 27).

CCNS Cape Cod National Seashore

EM Electromagnetic

IPEC Intergovernmental Panel on Climate Change

NPS National Park Service

NTAFB North Truro Air Force Base

TAZ Traffic Analysis Zone

USGS U.S. Geological Survey

WRAB Eastham Water Resources Advisory Board

Multiply By To Obtain

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

foot per day (ft/d) 0.3048 meter per day (m/d)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

foot squared per day (ft2/d) 0.0929 meters squared per day (m2/d)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

inch (in.) 25.4 millimeter (mm)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

mile (mi) 1.609 kilometer (km)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

pounds per cubic foot (lb/ft3) 16,018 milligrams per liter (mg/L)
Simulated Interaction Between Freshwater and 

Saltwater and Effects of Ground-Water 

Pumping and Sea-Level Change, Lower Cape Cod 

Aquifer System, Massachusetts

By John P. Masterson

Abstract

The U.S. Geological Survey, in cooperation with the 

National Park Service, Massachusetts Executive Office of 

Environmental Affairs, Cape Cod Commission, and the Towns 

of Eastham, Provincetown, Truro, and Wellfleet, began an 

investigation in 2000 to improve the understanding of the 

hydrogeology of the four freshwater lenses of the Lower Cape 

Cod aquifer system and to assess the effects of changing 

ground-water pumping, recharge conditions, and sea level on 

ground-water flow in Lower Cape Cod, Massachusetts.

A numerical flow model was developed with the computer 

code SEAWAT to assist in the analysis of freshwater and 

saltwater flow. Model simulations were used to determine water 

budgets, flow directions, and the position and movement of the 

freshwater/saltwater interface.

Model-calculated water budgets indicate that 

approximately 68 million gallons per day of freshwater 

recharge the Lower Cape Cod aquifer system with about 

68 percent of this water moving through the aquifer and 

discharging directly to the coast, 31 percent flowing through 

the aquifer, discharging to streams, and then reaching the coast 

as surface-water discharge, and the remaining 1 percent 

discharging to public-supply wells. The distribution of 

streamflow varies greatly among flow lenses and streams; in 

addition, the subsurface geology greatly affects the position and 

movement of the underlying freshwater/saltwater interface. 

The depth to the freshwater/saltwater interface varies 

throughout the study area and is directly proportional to the 

height of the water table above sea level. Simulated increases in 

sea level appear to increase water levels and streamflows 

throughout the Lower Cape Cod aquifer system, and yet 

decrease the depth to the freshwater/saltwater interface. The 

resulting change in water levels and in the depth to the 

freshwater/saltwater interface from sea-level rise varies 

throughout the aquifer system and is controlled largely by 

non-tidal freshwater streams. 

Pumping from large-capacity municipal-supply wells 

increases the potential for effects on surface-water bodies, 

which are affected by pumping and wastewater-disposal 

locations and rates. Pumping wells that are upgradient of 

surface-water bodies potentially capture water that would 

otherwise discharge to these surface-water bodies, thereby 

reducing streamflow and pond levels. Kettle-hole ponds, such 

as Duck Pond in Wellfleet, that are near the top of a freshwater 

flow lens, appear to be more susceptible to changing pumping 

and recharge conditions than kettle-hole ponds closer to the 

coast or near discharge boundaries, such as the Herring River. 
2 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Introduction

The ground-water lenses that constitute the Lower Cape 

Cod aquifer system (Nauset, Chequesset, Pamet, and Pilgrim 

lenses), are the sole source of drinking water for the Towns of 

Eastham, Wellfleet, Truro, and Provincetown, and the Cape 

Cod National Seashore (fig. 1). Increased land development and 

population growth have created concerns regarding both the 

quantity and the quality of ground water that is used for 

drinking water and that discharges to surface-water bodies 

and coastal areas throughout Lower Cape Cod.

These concerns described above include the effects of 

increased ground-water pumping on the position of the 

interface between freshwater and saltwater and on the amount 

of freshwater discharge to ponds, streams, and coastal areas. 

Ground-water discharge on Lower Cape Cod is the primary 

source of water for kettle-hole ponds and streams, and it also is 

a key component in the maintenance of the ecologically 

sensitive coastal embayments. Declines in water levels because 

of increases in ground-water withdrawals could have a 

detrimental effect on these natural resources. Small changes in 

water-table altitude can result in substantial decreases in 

ground-water discharge to streams and coastal embayments, 

and can substantially affect the shoreline position of the many 

kettle-hole ponds throughout Lower Cape Cod (Sobczak and 

others, 2003). 

Presently (2003), only the residents of Provincetown and 

small portions of Truro and Wellfleet are serviced by a publicwater supply system. The other residents of Lower Cape Cod 

obtain drinking water from shallow, small-capacity domesticsupply wells. As land development increases and wastewater 

continues to be returned to the aquifer through on-site, domestic 

septic systems, there is a growing concern that the increased 

amounts of non-point source contamination in the Lower Cape 

Cod aquifer system may adversely affect the existing water 

supply and may necessitate a shift from small-capacity 

domestic supplies to larger, more centralized municipal 

supplies (Sobczak and Cambareri, 1998). 

Federal, State, and local officials responsible for managing 

and protecting water resources are concerned that a shift to 

large-capacity, centralized municipal supplies may create the 

potential for unacceptable declines in water-table and pond 

altitudes, decreases in ground-water discharge to streams and 

coastal areas, and saltwater intrusion. In response to these 

concerns, the U.S. Geological Survey (USGS), in cooperation 

with the National Park Service, Massachusetts Executive Office 

of Environmental Affairs, Cape Cod Commission, and the 

Towns of Eastham, Provincetown, Truro, and Wellfleet began 

an investigation in 2000 to improve the understanding of the 

hydrogeology of the Lower Cape Cod aquifer system and to 

assess possible effects of proposed water-management 

strategies on Lower Cape Cod. 

This report describes the hydrogeology of the four flow 

lenses of the Lower Cape Cod aquifer system. A numerical 

ground-water-flow model was developed as part of this 

investigation to assist in the analysis of freshwater and saltwater 

flow for current and changing pumping and recharge 

conditions. Results from previous investigations that 

characterized the hydrogeology and ground-water flow of 

Lower Cape Cod, such as Guswa and LeBlanc (1985), LeBlanc 

and others (1986), Cambareri and others (1989), Masterson and 

Barlow (1996), Barlow (1996), Martin (1993), and Sobczak and 

Cambareri (1998), served as the foundation for the 

understanding of ground-water flow in the Lower Cape aquifer 

system. Results from these previous investigations were 

incorporated into the development and calibration of the 

ground-water-flow model developed for this investigation. 

The newly released computer program SEAWAT (Guo 

and Langevin, 2002) was used to provide information about 

regional-scale flow in the ground-water-flow lenses, including 

regional movement of the interface separating the freshwaterand saltwater-flow systems. Although detailed analyses of 

local-scale hydrologic conditions were beyond the scope of this 

investigation, the flow model may serve as the starting point for 

more detailed, site-specific investigations where local-scale 

models may be developed. 

The author thanks the members of the Lower Cape Cod 

Stakeholders Committee for their assistance and guidance 

throughout the duration of this investigation as well as the 

individuals from the following organizations who provided data 

or assisted in the aquisition of data during this investigation: 

Cape Cod Commission; National Park Service; Towns of 

Eastham, Provincetown, Wellfleet, and Truro; Barnstable 

County Board of Health; and Environmental Partners Group, 

Inc. The author also thanks USGS colleagues Ann Whealan and 

Timothy McCobb for their assistance in data collection and 

compilation, Stephen Garabedian for his guidance in solutetransport modeling, and Byron Stone for his assistance in 

interpreting the depositional history of the glacial sediments of 

Lower Cape Cod. 
Introduction 3

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

Chequesset 

Neck

Bound

Brook

Island

Pamet River

Herring River

Black Fish Cr.

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Duck

Pond

Pilgrim Lake

Pamet

flow lens

TSW-200

EGW-45

Pilgrim

flow lens

Chequesset

flow lens

Nauset

flow lens

Tide-control

structure

Griffin

Island

4

6

8

2

4

6

2

4

6

2

4

6

8

2

2

6

4

4

8

8

10

12

14

16

EGW-45

EXPLANATION

2

Study

area

Figure 1. Location of the four flow lenses of the Lower Cape Cod aquifer system and model-calculated water-table contours, 

Cape Cod, Massachusetts.
4 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Geologic Setting

The glacial deposits that constitute the Lower Cape Cod 

aquifer system consist of sediments that range in size from clay 

to boulders. Approximately 15,000 years ago during the late 

Wisconsinan glacial stage of the Pleistocene Epoch (Oldale and 

O’Hara, 1984), streams flowing from the coalescing lobes 

of the Cape Cod Bay and South Channel glacial ice sheets 

deposited the glacial sediments that now constitute Lower Cape 

Cod (fig. 2). These surficial deposits (fig. 3) overlie Paleozoic 

crystalline bedrock that ranges in altitude from about 450 ft 

below NGVD 29 in Eastham to more than 900 ft below NGVD 

29 in Truro (fig. 1) (Oldale, 1992). 

Depositional History

The sediments of the Lower Cape Cod aquifer system were 

deposited by meltwater from the retreating Cape Cod Bay and 

South Channel Lobe ice sheets as deltas prograded into a large 

glacial lake that formed in present-day Cape Cod Bay (Oldale, 

1992). Glacial Lake Cape Cod was dammed to the south and 

west by the older glacial deposits of upper and middle Cape Cod 

and to the north and east by the ice sheets. The glacial lake grew 

in size in the wake of the retreating ice sheets. 

The lake level of this glacial lake changed with time and 

that change is reflected, in part, in the altitude of the present-day 

land surface throughout Lower Cape Cod. The land-surface 

altitudes of the outwash plains represent the tops of the fluvial 

sediments deposited by braided rivers flowing from the ice 

lobes. The subsurface contact between the horizontal beds of 

river deposits and sloping beds of glaciolacustrine deposits 

indicates the lake stage that controlled the deltaic deposition. 

Oldale (1992) reports that the glacial lake stage was about 50 ft 

above the present sea level when the Wellfleet plain was 

deposited and less than 30 ft above present-day sea level when 

the Eastham plain was deposited. This deposition indicates that 

the stage of the glacial lake changed with time and that it was 

higher than the present-day sea level. 

The flat surfaces of the outwash plains are altered by the 

numerous kettle holes that were formed as collapse structures 

by the melting of buried blocks of ice stranded by the retreating 

ice lobes. These ice blocks, stranded directly on basal till and 

bedrock, subsequently were buried by prograding deltaic 

sediments. When the buried ice blocks melted, coarse sands and 

gravels collapsed into the resulting depressions. The kettle 

holes that intercept the water table now are occupied by 

kettle-hole ponds. 

Cape Cod

Bay lobe

Cape Cod

Bay lobe

South

Channel

lobe

South

Channel

lobe

Buzzards

Bay lobe

Buzzards

Bay lobe

A. Approximately 18,000 years ago B. Approximately 15,000 years ago

Schematic diagram, not to scale Modified from Oldale and Barlow (1986)

Figure 2. Ice recession and lobe formation in southeastern Massachusetts with respect to the present-day geography of 

the Cape Cod area. 
Geologic Setting 5

Qhw

Qep

Qs

Qd

Qd

Qb

Qdi

Qld2

Qlu

Qtp Qh af

Qwo

Qnh

af

Qb Qd

Qs

Qhw

Qdi

Qep

Qld2 Qlu

Qtp

Qh

Qwo

Qnh

Geologic Map UnitsPOST-GLACIAL DEPOSITS

Af - Artificial FillQd - Dune DepositsQb - Beach DepositsQsu - Sand and Gravel, UndifferentiatedQs - Marsh and Swamp Deposits

GLACIAL DEPOSITS

Qep - Eastham Plain DepositsQtp - Truro Plain DepositsQh - Highland Plain DepositsQwo - Wellfleet Plain DepositsQnh - Nauset Heights DepositsQlu - Cape Cod Bay Lake DepositsQld2 - Older Cape Cod Bay Lake DepositsQhw - Harwich Outwash Plain DepositsQdi - Dennis Ice-Contact Deposits

CORRELATION OF GEOLOGIC UNITS—LOWER CAPE COD (MODIFIED FROM OLDALE AND BARLOW, 1986)POST-GLACIAL DEPOSITS GLACIAL DEPOSITS

CAPE COD

BAY LOBE

DEPOSITS

SOUTH

CHANNEL

LOBE

DEPOSITS

HOLOCENE

PLEISTOCENE

DELTAIC DEPOSITS—FROM OLDESTTO YOUNGEST

1 WELLFLEET OUTWASH PLAIN2 TRURO OUTWASH PLAIN3 NAUSET OUTWASH PLAIN

1

2

3

70o15' 70o00'

42o

00'

41o

52'

30"

5 MILES 0

0 5 KILOMETERS

Figure 3. Surficial geology of Lower Cape Cod, Massachusetts (from Foster and Poppe, 2003; modified from Oldale and Barlow, 1986), and the 

depositional sequence of the Wellfleet, Truro, and Eastham outwash plains.
6 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

In addition, east-west-trending stream valleys, or pamets, 

were carved into the outwash-plain surfaces. These valleys 

formed possibly as a result of a process referred to as "spring 

sapping" (Oldale, 1992), in which ground-water springs 

intersect and erode up a gently sloping land surface. The 

gradual inland migration of the springs erodes the land and 

forms steep-headed channels. 

The deltaic sediments deposited in this glacial lake can 

be divided into topset, foreset, and bottomset beds (fig. 4) 

(B.D. Stone, U. S. Geological Survey, oral commun., 2001). 

The topset beds consist of glaciofluvial outwash of coarse 

sand and gravel deposited by braided rivers flowing from 

the ice lobes. The underlying foreset beds are glaciolacustrine 

sediments that consist mostly of medium to fine sand with some 

silt and are deposited subaqueously in a nearshore lake 

environment. The bottomset beds are glaciolacustrine 

sediments that consist of fine sand, silt, and clay and are 

deposited in an offshore lake environment. 

The sediments transported by meltwater streams flowing 

from the retreating ice lobes into Glacial Lake Cape Cod created 

three large deltas that constitute the Wellfleet, Truro, and 

Eastham outwash plains. The order in which these deltas were 

deposited was determined by the position and movement of the 

South Channel and Cape Cod Bay ice lobes (fig. 2). The 

Wellfleet plain is the oldest of the three large plains, the Truro 

plain is intermediate, and the Eastham plain is the youngest 

(fig. 3). The sediment sources for these plains generally were 

north and east of the present location of Lower Cape Cod in 

what is now the Atlantic Ocean. A smaller outwash plain, the 

Highland plain, formed after the Wellfleet plain but before the 

Truro plain in a small glacial lake between the Wellfleet plain 

and the South Channel ice sheet (Oldale, 1992). 

Once the ice sheets retreated, sea level began to rise and 

erosion of the glacial Cape Cod shoreline began. The original 

heads of the outwash plains were eroded back to the present-day 

shoreline. The erosion that resulted from sea-level rise left 

behind the distal portions of the outwash plains that constitute 

three (Nauset, Chequesset, and Pamet) of the four flow lenses 

of the Lower Cape Cod aquifer system.

The Pilgrim flow lens was formed by the progradation of a 

post-glacial barrier spit of sand deposited by the prevailing 

ocean currents as sea level rose during the Holocene Epoch. 

These sands consist of eroded glacial material deposited on top 

of older glaciolacustrine sediments that were deposited in the 

offshore environment present during the deposition and 

subsequent erosion of the Truro plain (Zeigler and others, 1965; 

Uchupi and others, 1996). The Provincetown spit and resulting 

sand dunes continue to migrate today and parts of the outwash 

plains that are exposed to the Atlantic Ocean continue to be 

eroded at a retreat rate as high as 7 to 11 ft/yr (Oldale, 1992).

Geologic Framework

The general trends in sediment distribution within 

deltaic deposits are coarsening upward and fining with 

distance from the sediment source. This general trend is 

illustrated in lithologic sections reported by Masterson and 

others (1997) for western Cape Cod. On Lower Cape Cod, 

however, the distance of the present-day outwash plains from 

the now-eroded sediment sources (heads of the outwash plains) 

is reflected in the subsurface grain-size distributions in the flow 

lenses and accounts for the large differences in hydraulic 

properties and ground-water-flow patterns between the lenses 

(B.D. Stone, U.S. Geological Survey, oral commun., 2001). 

For example, in the Pamet flow lens, which consists of 

Wellfleet and Truro plain deposits, the sediment source of 

the glacial deposits is much closer than the sediment source 

for the deposits of the Eastham plain that constitute the 

Nauset flow lens (fig. 3). The differences in distance from 

the sediment sources between the outwash plains is reflected 

in the subsurface lithology. In the Pamet lens, lithologic borings 

show thick sequences of fine, medium, and coarse 

sand and gravel that extend several hundred feet below land 

surface. Conversely, lithologic borings in the Nauset lens show 

a thin layer (less than a hundred feet) of fine, medium, and 

coarse sand and gravel underlain by a thick sequence of layered 

silts and clays. 
Hydrologic System 7

EXPLANATION

HYDROGEOLOGIC UNITS

Primarily coarse-grained deposits

Primarily fine-grained deposits

FORESET BEDS FORESET BEDS FORESET BEDS

BEDROCK BOTTOMSET BEDS B

BOTTOMSET BEDS OTTOMSET BEDS

TOPSET BEDS

GLACIAL LAKE

Figure 4. Schematic diagram of deltaic deposits prograding into a glacial lake, including topset, foreset, and bottomset 

deposits (modified from Smith and Ashley, 1985).

USGS Coastal and Marine scientists (Foster and Poppe, 

2003) used marine seismic radar off the coasts of Cape Cod Bay 

and the Atlantic Ocean near North Eastham and South Wellfleet 

to determine the offshore extent of the glaciolacustrine deposits 

observed in the onshore lithologic borings. On the basis of the 

seismic profiles, they concluded that the thick deposits of 

layered silts and clays observed in the onshore lithologic boring 

extend beyond the eastern and western shores of the Nauset 

flow lens, and, therefore, are extensive beneath this flow lens. 

This discussion on the glacial history and geologic 

setting of Lower Cape Cod is presented in this report to 

provide a cursory description of the geologic framework 

that served as the foundation for the depositional model 

of the glacial sediments incorporated into the ground-waterflow model developed for this investigation. For more detailed 

descriptions and analyses of the glacial history and geologic 

framework of Cape Cod, readers are referred to the following 

reports: Woodworth and Wigglesworth (1934); Kaye (1964); 

Zeigler and others (1965); Oldale and O’Hara (1984); Oldale 

and Barlow (1986); Oldale (1992); and Uchupi and others 

(1996).

Hydrologic System

The glacial sediments that underlie the towns of Eastham, 

Wellfleet, Truro, and Provincetown constitute the Lower Cape 

Cod aquifer. The freshwater flow in this aquifer is bounded 

laterally and below by saltwater (fig. 5), and it often is referred 

to as an aquifer system because it consists of four freshwater 

flow lenses—Nauset, Chequesset, Pamet, and Pilgrim (Horsley 

and others, 1985). The flow lenses were characterized and the 

aquifer system was analyzed under changing hydrologic 

conditions by use of the ground-water-flow model developed 

for this investigation. A detailed discussion of the development 

and calibration of this model is provided in the appendix. 
8 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

SALINE GROUND SALINE GROUND

WATER

SALINE GROUND SALINE GROUND

WATER

WATER TABLE

LAKE

SALINE GROUND

WATER

SALINE GROUND

WATER

BEDROCK

Schematic diagram, not to scale

FRESH GROUND

WATER

Figure 5. Schematic diagram of the Lower Cape Cod aquifer system, Cape Cod, Massachusetts 

(modified from Strahler, 1972).

Simulation of Ground-Water 

Flow in the Lower Cape Cod 

Aquifer System

The freshwater flow lenses of the Lower Cape Cod aquifer 

system consist of four large water-table mounds separated from 

one another by inter-lens surface-water-discharge areas. Under 

current conditions, the four flow lenses are hydraulically 

independent of one another (fig. 1). Ground water flows radially 

from the tops of the ground-water mounds toward the coast and 

the inter-lens surface-water-discharge areas; flow from one lens 

does not discharge to another lens. 

The inter-lens discharge areas are tidally affected marshes 

and streams under natural hydrologic conditions. However, 

tide-control structures installed in 1868 by the State of 

Massachusetts and the Towns of Provincetown and Truro to 

restrict the inland movement of saltwater at Pilgrim Lake and 

the Pamet River (fig. 1) changed these ecosystems from salt to 

freshwater. The Cape Cod Commission (Eichner and others, 

1997) and the National Park Service have studied the effects of 

tide-control structures on flow in these areas and the possible 

hydrologic effects of returning these ecosystems to their natural 

state. 

The primary source of freshwater for the Lower Cape 

Cod aquifer system is precipitation. The weather stations 

in Provincetown (National Oceanic and Atmospheric 

Administration, 2001, 1948–92) and South Truro (National 

Atmospheric Deposition Program Station: MA-01, 1980–2000) 

report an average rainfall rate of about 42 in/yr from 1948 to 

2000. Previous ground-water-modeling investigations on 

western Cape Cod (Masterson and others, 1998) indicate that, 

of the 42 in/yr of rainfall, 45 percent is removed by evaporation 

and plant transpiration before reaching the water table. The 

remaining freshwater that enters the aquifer system is referred 

to as aquifer recharge. 

Aquifer recharge rates vary from year to year in response 

to changes in annual precipitation. These rates also may vary 

from flow lens to flow lens because of differences in land use, 

vegetation, depth to the water table, and rainfall; however, 

determining the spatial and temporal changes in aquiferrecharge rates was beyond the scope of this investigation. Thus, 

for this investigation, it was assumed that, on average, 24 in/yr, 

or 55 percent of the average annual precipitation, reaches the 

aquifer system as recharge throughout the study area. 
Simulation of Ground-Water Flow in the Lower Cape Cod Aquifer System 9

Ground-Water Recharge Areas 

All of the water that enters the aquifer system as recharge 

ultimately discharges to pumped wells, streams, coastal 

marshes, and beaches. Some of this water may flow through 

kettle-hole ponds on its way to these discharge areas. The 

source of water to these discharge points, or receptors, can be 

determined by mapping the area at the water table that, 

multiplied by the recharge rate, satisfies the total flow to the 

receptor. The concept of the source of water to a hypothetical 

pumped well is illustrated schematically in figure 6. This 

concept can be applied to any hydrologic feature that receives 

ground-water discharge, such as kettle-hole ponds, streams, and 

coastal areas (Masterson and Walter, 2000). The discharge 

locations of all water that enters the aquifer system can be 

determined once the recharge areas to all hydrologic features 

are delineated (fig. 7). 

The size of the recharge areas to various hydrologic 

features is proportional to the amount of water that discharges 

to these features when a spatially consistent recharge rate 

has been applied. For instance, the areas shown on figure 7 

that delineate the sources of water to Cape Cod Bay and the 

Atlantic Ocean represent a large percentage of the total study 

area. The model-calculated water budget (table 1) shows 

that nearly 68 percent of the total flow through the aquifer 

system discharges to the coast as direct ground-water discharge. 

About 31 percent of the total flow reaches the coast as groundwater-derived streamflow. Pumping from municipal-supply 

wells captures about 1 percent of the recharge, all of it from the 

Pamet flow lens. 

Water Budget

All of the freshwater that flows through the Lower Cape 

Cod aquifer system is derived from aquifer recharge. A modelsimulated recharge rate of 24 in/yr yields a total freshwater flow 

through the aquifer system of about 67.3 Mgal/d. An additional 

0.8 Mgal/d of water is returned to the aquifer in the Pilgrim flow 

lens from water pumped in the Pamet flow lens and discharged 

to the Pilgrim flow lens as wastewater discharge from septic 

systems. Therefore, the total freshwater flow through the 

aquifer system is about 68 Mgal/d (table 1). 

The total water budget for the Lower Cape Cod aquifer 

system can be subdivided by individual flow lenses (table 1). 

Subdividing the water budget by flow lens provides a better 

understanding of the distribution of flow to the various 

hydrologic features than can be obtained from the total water 

budget for the entire aquifer system. For instance, the total 

amount of ground-water discharge to streams is about 21 

Mgal/d, or 31 percent of the total water budget for the Lower 

Cape Cod aquifer system, yet nearly 60 percent of that 

streamflow is in the Herring River, which bisects the 

Chequesset flow lens (fig. 1). 

Ground-water withdrawals for public supply only account 

for about 1 percent of the the total water budget for the entire 

aquifer system; however, all of that pumping occurs in the 

Pamet flow lens and that pumping constitutes about 7 percent of 

the total budget of that flow lens. This pumped water is the 

primary source of drinking water for the town of Provincetown, 

parts of the town of Truro, and some National Park Service 

facilities in the Provincetown area. 

A small-capacity municipal supply also is pumping 

water from the Chequesset flow lens. This well services 

approximately 30 residences in the Coles Neck area of 

Wellfleet and pumps on average about 7,000 gal/d. Because this 

well is pumping at a low rate compared to the total flow in 

aquifer system, it is not included in the overall water budgets for 

the Lower Cape Cod aquifer system or the Chequesset flow 

lens. This is also the case for the domestic wells from which 

many of the residents of Lower Cape Cod obtain their drinking 

water because the water pumped from and returned to the same 

part of the aquifer resulted in no effect on the flow system.

Figure 6. Area contributing recharge to a pumping well in a 

simplified, hypothetical ground-water-flow system (modified 

from Reilly and Pollock, 1993).
10 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

2

4

2

ro

Cape Cod Bay

Atlantic Ocean

at Pond

ow Pond

Ryder Pond

Great P

Herring Po

Minister P

Depot Po

GROUND-WATER RECHARGE AREAS TO:

Cape Cod Bay

Atlantic Ocean

Streams

Coastal Wetlands

Ponds

Wells

Pilgrim Lake

Pilgrim Lake Wetlands

Nauset Marsh

Town Cove

Boat Meadow

EXPLANATION

WATER-TABLE CONTOUR—Contour interval

is 2 feet. Datum is above local sea level.

MODEL SECTION

PUMPING WELL

2

A'

B

B'

B'

2

Chequesset

Neck

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Bound Bro

Island

South

Hollow

Kn

Cro g

North Truro AFB well No. 4

North Truro AFB well No. 5

Coles

Neck

Figure 7. Delineation of ground-water-recharge areas to public-supply wells, ponds, streams, and coastal areas for current 

(2002) average pumping and recharge conditions, Cape Cod, Massachusetts. Sections A-A′ and B-B′ shown in figures 9 and 10, 

respectively. 
Simulation of Ground-Water Flow in the Lower Cape Cod Aquifer System 11

Altitude and Configuration of 

Water-Table Mounds 

The altitude and configuration of the water-table mounds 

in the Lower Cape Cod aquifer system are affected by factors 

such as surface-water bodies, the geologic framework, and 

changing pumping and recharge conditions. In general, ground 

water flows radially from the highest point of the water table 

toward the coast and the inter-lens discharge areas. Water 

entering near the center of the water-table mound travels deeper 

through the aquifer system than water that recharges near the 

coast (fig. 5). 

The altitude of the water-table mounds differ (fig. 1) 

from about 16 ft above NGVD 29 in the Nauset flow lens to 

about 6 ft above NGVD 29 in the Pilgrim flow lens. The 

Chequesset flow lens, which receives the most recharge of 

the four flow lenses, has a maximum altitude of about 9 ft 

above NGVD 29; this altitude is about 7 ft lower than that 

of the Nauset flow lens. The differences among the maximum 

altitudes and configurations of the flow lenses result from 

differences in the distribution of surface-water bodies and in the 

geologic framework. 

Interaction Between Ground and 

Surface Waters 

The water-table altitude and configuration of the 

Chequesset flow lens is affected more by surface-water 

discharge than the other flow lenses of the Lower Cape Cod 

aquifer system. In the Chequesset flow lens the Herring River 

and its tributaries (fig. 1) occupy a large area in the western part 

of the flow lens and drain a large percentage of the total flow 

in the lens toward Wellfleet Harbor as surface-water discharge. 

The model-calculated streamflow in the Herring River and 

associated tributaries is about 7.4 Mgal/d, or about 30 percent 

of the total flow through the Chequesset flow lens. 

The Herring River and its tributaries affect the surrounding 

water table because the altitude of the stream-channel bottom 

does not change with time. The water table in the vicinity of 

the streams cannot rise appreciably above the stream channel 

because, as the water-table altitude increases in response to 

changing recharge conditions, ground-water discharge to 

the stream increases. In the absence of streams, increases 

in recharge would result in a corresponding increase in 

ground-water levels. 

The Chequesset flow lens, which is separated from the 

Pamet flow lens to the north by the Pamet River and the Nauset 

flow lens to the south by Blackfish Creek, can be subdivided 

further. The Herring River complex subdivides the flow lens, 

creating two large flow lenses and three small flow lenses at 

Bound Brook Island, Griffin Island, and the area occupied by 

Wellfleet center (fig. 1). Although these five flow lenses are 

hydrologically independent of each other under present 

conditions, they were considered to be part of the larger 

Chequesset flow lens for the purpose of this investigation. 

Kettle-hole ponds also affect the configuration of the 

water table in the flow lenses. The ponds, like streams, are 

hydraulically connected to the ground-water-flow system, 

causing pond levels and streamflows to fluctuate with groundwater levels. Pond levels fluctuate less than surrounding 

ground-water levels because ponds have substantially larger 

storage capacities than the aquifer. The ponds of Lower Cape 

Cod are areas of net recharge because the annual precipitation 

rate (42 in/yr) exceeds the annual potential evaporation rate 

from pond surfaces (28 in/yr) (Farnsworth and others, 1982) 

for average annual flow conditions. For periods of extended 

droughts, however, substantially more water can be lost 

from pond surfaces creating ground-water sinks during these 

extended drought periods. Information on how kettle-hole 

ponds are simulated in the ground-water-flow model is reported 

in the appendix. 

The recharge areas for kettle-hole ponds are delineated 

in a manner similar to streams and coastal areas because the 

upgradent side of a pond acts as a ground-water discharge zone 

(fig. 7). Unlike the discharge into streams, however, water that 

discharges to a pond does not necessarily result in net removal 

of water from the aquifer because the water mixes within the 

pond and either passes through the downgradient side of the 

pond and re-enters the aquifer or moves directly into outflowing 

streams. 

In the Lower Cape Cod aquifer system there are clusters of 

kettle-hole ponds in the center of the Chequesset flow lens and 

in the lower part of the Nauset flow lens (fig. 7). The pond 

cluster consisting of Snow, Great, and Ryder Ponds in the center 

of the Chequesset flow lens has smaller recharge areas than the 

pond cluster consisting of Minister, Great, Depot, and Herring 

Ponds in the lower part of the Nauset flow lens (fig. 7). This 

difference is due, in part, to the size and depths of the ponds, but 

also to the pond locations with respect to the top of the watertable mound. 

Table 1. Model-calculated hydrologic budget for the four flow 

lenses of the Lower Cape Cod aquifer system under current (2002) 

pumping and recharge conditions, Cape Cod, Massachusetts.

[Inflow: Consists of recharge from precipitation and wastewater. All values in 

million gallons per day]

Flow lens Inflow

Outflow

Coast Streams Wells Total for 

each lens

Nauset 19.0 14.0 5.0 0.0 19.0

Chequesset 24.2 11.9 12.3 .0 24.2

Pamet 12.4 8.8 2.7 .9 12.4

Pilgrim 12.5 11.4 1.1 .0 12.5

Total 68.1 46.1 21.1 0.9 68.1
12 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Because the ponds in the Chequesset flow lens are near the 

top of the water-table mound, their contributing areas are small 

and the total flow through the ponds is low. In comparison, the 

ponds in the lower part of the Nauset flow have larger 

contributing areas and higher flow (fig. 7). This difference in 

the position of the ponds within the flow system may affect how 

much water moves through the ponds and how the ponds 

respond to changing stress conditions, such as changing 

pumping and recharge conditions.

Freshwater ponds also are in the Pilgrim lens but they are 

of a different origin than the kettle-hole ponds in the Chequesset 

and Nauset flow lenses. The ponds on the Pilgrim lens were not 

formed by the collapse of melting buried blocks of ice, but 

rather from the more recent flooding of wetlands in lowland 

areas where the land surface has been intercepted by the 

underlying water table (B.D. Stone, U.S. Geological Survey, 

oral commun., 2001). Recharge areas to these ponds were 

delineated in a manner similar to that used to calculate the 

contributing areas to the kettle-hole ponds. Further 

investigation, however, would be needed to determine if the 

pond-bottom sediments in the flooded wetlands differ from the 

pond-bottom sediments of the kettle-hole ponds. Simulated 

differences in pond-bottom sediments have been shown to have 

little effect on model-calculated pond levels, but can have a 

large effect on the amount of flow through the ponds, and 

therefore, contributing areas to ponds (Walter and others, 

2002). 

Controls of Hydrogeologic Framework

Another explanation for the differences among the watertable altitudes in the flow lenses can be attributed to the 

differences in the hydrogeologic framework among the flow 

lenses. For example, the Nauset flow lens, unlike the 

Chequesset flow lens, consists of thick deposits of lowpermeability layers of silt and clay at depth. The presence of this 

low-permeability material in the Nauset flow lens lowers the 

overall transmissivity of the aquifer and alters vertical flow 

paths such that proportionally more water flows through the 

thinner, overlying, more permeable sediments than the thicker, 

underlying, less permeable sediments. The focus of flow in the 

thinner, more permeable sediments results in higher water-table 

altitudes than would be expected if the more permeable 

sediments extended deeper into the aquifer. 

Simulated Interaction Between 

Freshwater- and Saltwater-Flow 

Systems

Freshwater flow in the Lower Cape Cod aquifer system is 

bounded below by saltwater rather than truncated by bedrock as 

is the case on western Cape Cod (Masterson and Barlow, 1996). 

The reason for the bounding by saltwater is that the flow lenses 

of Lower Cape Cod are much smaller in size than those of 

western Cape Cod, the depth to bedrock is greater in general 

under Lower Cape Cod than western Cape Cod, and because 

of the smaller land area, less recharge from precipitation is 

available to extend the freshwater lenses deep enough to 

intersect bedrock. 

Depths to the freshwater/saltwater interface differ among 

the flow lenses; these depths are directly proportional to altitude 

of the overlying water table. If the altitude of the water table 

above sea level (zw) is lowered by 1 ft, the depth to the 

freshwater/saltwater interface (zs) decreases by 40 ft (Ghyben, 

1888; Herzberg, 1901): zs = 40zw. The Ghyben-Herzberg 

relation is based on the density difference between fresh and salt 

waters and is a general approximation, subject to many 

simplifying assumptions, of the actual interaction between 

freshwater and saltwater flow. The relation does provide 

insight, however, into the differences in the depths to the 

freshwater/saltwater interfaces throughout the Lower Cape Cod 

aquifer system. 

Field measurements show that the depth to the 

freshwater/saltwater interface is about 350 ft below NGVD 29 

at test site EGW-45 in the Nauset flow lens (fig. 1) (Barlow, 

1996); in the Pamet flow lens, the altitude of the water table is 

much lower and the depth to the interface is about 250 ft below 

NGVD 29 at test site TSW-200 (fig. 1) (LeBlanc and others, 

1986). 

Because the Ghyben-Herzberg relation is approximate and 

subject to simplifying assumptions, and the expense of drilling 

monitoring wells to the freshwater/saltwater interface 

throughout the study area is high, a numerical model was 

developed to characterize the position and movement of the 

freshwater/saltwater interface throughout the Lower Cape Cod 

aquifer system. The numerical model SEAWAT (Guo and 

Langevin, 2002) that simulates variable-density, transient 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 13

ground-water flow in three dimensions was used for this 

investigation. The model development and calibration is 

described in detail in the appendix. 

The model-calculated three-dimensional representation of 

the depth to the freshwater/saltwater interface shows how the 

depth to this interface changes throughout the Lower Cape Cod 

aquifer system (fig. 8). The depth to the freshwater/saltwater 

interface is greatest, up to 400 ft below NGVD 29, beneath the 

tops of the water-table mounds and shallowest at the inter-lens 

discharge areas. 

Effects of Surface-Water Bodies

A south-north vertical section from the center of the 

Chequesset flow lens across the Pamet River to the center of the 

Pamet flow lens (fig. 9) shows that the depth to the interface is 

about 300 ft below NGVD 29 beneath the center of the 

Chequesset flow lens. This depth decreases to about 100 ft 

below NGVD 29 beneath the Pamet River and then increases 

to the north to about 250 ft below NGVD 29 beneath the center 

of the Pamet lens, as calculated by the numerical model. 

The decrease in depth to the freshwater/saltwater interface 

beneath the Pamet River is a function of the low water-table 

altitude along the stream reach (about 2 ft above NGVD 29) and 

the fact that ground-water discharge to the river reaches the 

coast as surface-water flow and, therefore, is removed from the 

ground-water-flow system. 

Surface-water discharge also affects the position of the 

freshwater/saltwater interface in the Chequesset flow lens 

in the vicinity of the Herring River. A simulated west-east 

vertical section across the Chequesset lens (fig. 10) shows an 

asymmetric shape to the position of the freshwater/saltwater 

interface with the depth to the interface greatest beneath 

the eastern side of the flow lens. This asymmetric geometry of 

the interface can be explained by the water-table configuration 

and the distribution of ground-water recharge areas shown on 

figure 7. 

The map of the ground-water recharge areas for the Lower 

Cape Cod aquifer system (fig. 7) shows that a large area on 

the west side of the Chequesset flow lens contributes water to 

the Herring River and its tributaries. This water is lost to the 

ground-water-flow system because it discharges into the stream 

channel and is transported directly to Wellfleet Harbor as 

surface-water discharge. On the east side of the Chequesset 

flow lens, ground water also discharges to surface-water 

bodies; however, in the case of the kettle-hole ponds, most 

of the water flows through the ponds, re-enters the 

aquifer, and then discharges to the Atlantic Ocean as direct 

ground-water discharge. It is this difference in how the recharge 

reaches the coast and the effect of streams on the water-table 

mounds that accounts for the asymmetry in the position of the 

freshwater/saltwater interface in the Chequesset flow lens.

Pilgrim

flow

lens

Pamet

flow lens

Pilgrim Lake

South

Hollow

well field

Pamet

River

Herring

River

Black

Fish

Creek

Cape Cod

Bay

Chequesset

flow lens

Nauset

flow

lens

70o15' 70o00'

42o

00'

41o

52'

30"

0 5 MILES

0 5 KILOMETERS

Figure 8. Model-calculated delineation of the boundary 

between freshwater and saltwater beneath the Lower Cape Cod 

aquifer system, Cape Cod, Massachusetts.
14 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

VERTICAL SCALE GREATLY EXAGGERATED

0 2

0 2 KILOMETERS

MILES EXPLANATION

FRESHWATER ZONE

TRANSITION ZONE

SALTWATER ZONE

South Hollow Wellfield South North Pamet River

0

 50

 100

 150

 200

 250

 300

 350

400

450

500

A' A

CHEQUESSET 

FLOW LENS

PAMET FLOW 

LENS

DEPTH, IN FEET BELOW NGVD 29

Figure 9. Model section showing the model-calculated boundary between freshwater and saltwater 

flow, Lower Cape Cod, Massachusetts. Section lineA-A′ shown on figure 7.

Effects of Ground-Water Pumping

Ground-water pumping also can affect the position and 

movement of the freshwater/saltwater interface. Because the 

flow lenses are bounded laterally and underlain by saltwater, 

there is concern for the potential of both lateral intrusion and 

upconing of saltwater from pumping. A pumping well near the 

coast can draw saltwater in laterally and a pumping well in 

the center of the peninsula can pull saltwater up from below 

(fig. 11). Saltwater intrusion is of greater concern in the Pamet 

flow lens than the other lenses because nearly all of the 

pumping for public supply in Lower Cape Cod takes place in 

the Pamet flow lens. 

Within the Pamet flow lens are three active well fields; 

Knowles Crossing, South Hollow, and the North Truro Air 

Force Base (fig. 12). These well fields are the primary water 

supply for the residents of Provincetown, parts of North Truro, 

and for various National Park Service facilities on the Cape Cod 

National Seashore. The South Hollow well field (also known as 

the Paul Daley well field) has been operating since 1955 and 

consists of eight pumping wells that currently (2002) provide, 

on average, 0.57 Mgal/d of water. The Knowles Crossing well 

field (also known as the Old Truro well field) is the oldest well 

field on Cape Cod and has been operating since 1907. This well 

field consists of two pumping wells that currently (2002) 

provide, on average, 0.20 Mgal/d. 

The third well field in the Pamet flow lens consists of two 

pumping wells (No. 4 and 5) on the site of the decommissioned 

North Truro Air Force Base (NTAFB), which is now part of the 

Cape Cod National Seashore. This well field was part of the 

water-supply system for the North Truro Air Force Base and, 

since 1978, has provided water to Provincetown to help the 

town meet its drinking-water demand. Currently, water is being 

withdrawn from these wells during a 6-month period from June 

through November for an average daily pumping rate of 

0.16 Mgal/d for the year. 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 15

West Bound Brook Herring River East

0

 50

 100

 150

 200

 250

 300

 350

400

450

500

B B'

VERTICAL SCALE GREATLY EXAGGERATED

0 2

0 2 KILOMETERS

MILES

CHEQUESSET FLOW LENS

EXPLANATION

FRESHWATER ZONE

TRANSITION ZONE

SALTWATER ZONE

DEPTH, IN FEET BELOW NGVD 29

Figure 10. Model section showing the model-calculated boundary between freshwater and saltwater 

flow, Lower Cape Cod, Massachusetts. Section line B-B′ shown on figure 7.

A fourth well field consisting of one well, Cape Cod 

National Seashore Site No. 4 (CCNS No. 4, fig. 12), was 

pumped seasonally from 1978 to 1985 to provide additional 

drinking water to Provincetown while pumping was reduced 

at the South Hollow well field. CCNS No. 4 was pumped 

from May to October for an average daily pumping rate of 

0.25 Mgal/d for the year. Information on pumping rates and 

pumping duration for all of the pumping wells in the Pamet 

flow lens are detailed in Provincetown’s water-managementplanning report (Environmental Partners Group, 2002). 

The threat of saltwater intrusion at the Knowles Crossing 

well field, which is about 1,000 ft inland of Cape Cod Bay, has 

concerned Provincetown water suppliers for many years. Until 

1955, this well field was the primary source of water for the 

residents of Provincetown. As population increased, the 

demand for drinking water increased; the average daily 

withdrawal rate from this well field increased from about 

0.16 Mgal/d in 1907 to 0.49 Mgal/d in 1954. This increase in 

water demand resulted in saltwater intrusion at the well field 

and a subsequent reduction to the current (2002) daily rate of 

0.20 Mgal/d. 

In 2000, the town of Provincetown began installing a 

number of deep monitoring wells at the three well fields to 

determine the depths of the freshwater/saltwater interface 

(Environmental Partners Group, 2002). Three deep monitoring 

wells were installed alongside the three pumping wells at 

Knowles Crossing. As part of this investigation, borehole 

geophysical electromagnetic (EM) and gamma logs were 

collected for the purpose of determining the depth to the 

freshwater/saltwater interface and for monitoring the interface 

position for seasonal changes. 

The EM conductivity logs collected at the Knowles 

Crossing well field show that the depth to the transition between 

fresh and salt waters increases from about 60 ft below 

NGVD 29 at the westernmost well (KC-1) to about 100 ft below 

NGVD 29 at the easternmost well (KC-3) (fig. 13). The natural 

gamma logs (fig. 13) show the presence of fine-grained 

sediments that were identified by the driller to be clay 

(Environmental Partners Group, 2002). The clay lenses appear 

to have affected the position of the freshwater/saltwater 

interface beneath the well field such that saltwater has laterally 
16 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

PUMPING

WELLS

PUMPING

WELLS

FRESHWATER FRESHWATER

SALTWATER SALTWATER

WATER TABLE

SALTWATER

BEDROCK

Schematic diagram, not to scale

PUMPING

WELLS

FRESHWATER

Figure 11. Schematic diagram of the Lower Cape Cod aquifer system showing lateral and vertical 

saltwater intrusion in response to ground-water pumping (modified from Strahler, 1972).

intruded above the clay lens at about 70 ft beneath pumping 

wells KC-1 and KC-2. This intrusion, based on the available 

geophysical data, is shown schematically on figure 14. 

In January 2001, the USGS, in cooperation with the town 

of Provincetown, installed an automated, real time, waterquality-sampling system, known as "Robowell" (Granato and 

Smith, 2002), in two monitoring wells above and below the clay 

lens near the KC-2 pumping well. Results of this real-time 

sampling show that the specific conductance of water in the 

shallow well (TSW-260) was nearly twice that in the deep well 

(TSW-259) and that the specific conductance of water in the 

shallow well showed a greater response to changes in pumping 

and recharge than that of the deeper well (fig. 15). This 

information indicates that the source of saltwater intrusion at 

the Knowles Crossing well field was most likely the result of 

lateral encroachment along the top of the clay layer rather than 

upconing beneath the well field. In response to this information, 

Provincetown replaced their three-well vacuum system in 2001 

with a two-well submersible system at the locations of KC-2 

and KC-3 and removed KC-1, the pumping well nearest to the 

coast (Environmental Partners Group, 2002). 

With water demand increasing and the total yield at 

Knowles Crossing limited by saltwater intrusion, Provincetown 

developed a new public-supply site in 1955 at South Hollow 

(fig. 12). The South Hollow well field served as the primary 

water-supply source for Provincetown until 1978 when gasoline 

was spilled in the vicinity of the well field (Environmental 

Partners Group, Inc., 2002). As a precautionary measure, the 

town limited withdrawals at the well field to avoid pumping 

contaminated water. This well field was either not pumped 

or pumped at a reduced rate from 1978 to 1985. Pumping 

resumed at full capacity by 1986 and currently (2002) averages 

0.57 Mgal/d. 

The ground-water model was used to simulate annual 

average pumping at the South Hollow well field from 1955 to 

present (2002) and calculate the change in the position of the 

freshwater/saltwater interface with time (fig. 16). The results 

indicate that, before the start of pumping in 1955, the depth to 

the freshwater/saltwater interface beneath the South Hollow 

well field was about 230 ft below NGVD 29 and that the 

interface has risen about 43 ft to its current position (2002) at 

about 187 ft below NGVD 29. 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 17

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Chequesset 

Neck

Pamet River

Herring R.

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

1

2

3

4 5

6

8

7

9

10

11

12

13

14

15

17 16

18

19

20

21

22

23

EXPLANATION

1

South Hollow well field

North Truro Air Force Base well No. 4

North Unionfield site North Truro Air Force Base well No. 5

CCC-5 site

Coles Neck Well

Boy Scout

Camp site

Wellfleet by

the Sea site 

Roach site

Water District G site

Knowles

Crossing

well field

CCNS No. 4 site

Hatches Creek

at West Road

Sunken Meadow

Long

Pond

Dyer

Pond

Great

Pond

Sewageinfiltration

beds

Auxillary

well site

Duck

Pond

L

River ittlePamet

Figure 12. Locations of existing (2002) and proposed public-supply wells and Traffic Analysis Zones, Lower Cape Cod, 

Massachusetts.
18 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

NATURAL GAMMA, IN

COUNTS PER SECOND

NATURAL GAMMA, IN

COUNTS PER SECOND

NATURAL GAMMA, IN

COUNTS PER SECOND

EM CONDUCTIVITY, IN MILLISIEMENS PER METER

ALTITUDE, IN FEET BELOW NGVD 29

(KC-1)

KNOWLES CROSSING- 1

(KC-2)

KNOWLES CROSSING- 2

(KC-3)

KNOWLES CROSSING- 3

-50 150 350 550 750

-100

-75

-50

-25

0

0

150 350 550 750

-100

-75

-50

-25

0

-50 150 350 550 750

-125

-50

-125

-100

-75

-50

-25

0

-125

50 100 150 0 50 100 150 0 50 100 150

EXPLANATION

EM CONDUCTIVITY, MEASURED 

SEPTEMBER 2000

NATURAL GAMMA

PUMPING WELL AND SCREEN 

INTERVAL

Figure 13. Profiles of natural gamma and electromagnetic (EM) geophysical logs at the Knowles Crossing well field, 

North Truro, Massachusetts, measured in September 2000.

The actual depth to the freshwater/saltwater interface in 

1955, before the start of pumping, is not known, and, therefore, 

this depth cannot be used to determine the accuracy of the 

model-calculated position of the freshwater/saltwater interface 

for prepumping conditions. Field measurements of the 

freshwater/saltwater interface position were made in 2001 as 

part of the deep-monitoring-well-installation and sampling 

program and provide data on the current position of the 

freshwater/saltwater interface beneath the Knowles Crossing, 

South Hollow, and North Truro Air Force Base well fields. 

At the South Hollow well field near the center of the Pamet 

flow lens (fig. 12), three deep monitoring wells were installed 

at both ends and at the middle of the well field, which consists 

of eight pumping wells. The EM log collected in the well 

at the westernmost end of the well field (SH-1) indicates a thick 

transition zone from about 85 ft to 175 ft below NGVD 29 

(fig. 17). Conversely, the EM log collected in the well at 

the easternmost end of the well field (SH-3) indicates a thin 

transition from 165 ft to 180 ft below NGVD 29 (fig. 17). 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 19

Saltwater

Land surface

Water table

Freshwater

Cape Cod Bay

Fine-grained unit

?

? ?

KC1 KC2 KC3

Schematic diagram, not to scale

TSW-260

TSW-259

Figure 14. Schematic diagram showing lateral and vertical saltwater intrusion beneath the Knowles Crossing well field, North 

Truro, Massachusetts.

Possible explanations for the thick transition zone at SH-1 

include the following. (1) The proximity of this well to Route 6 

may have resulted in road salt contamination. (2) The vacuum 

system at this well field in operation from 1955 to 2000 may 

have resulted in proportionally more water pumped from 

the westernmost well than the other wells; this additional 

withdrawal of water may have caused the freshwater/saltwater 

interface to rise more in this part of the well field. (3) The well 

field is oriented within the flow lens such that the western end 

is closer to the coast than the eastern end and, therefore, the 

transition zone on the west side of the well field may be thicker 

and shallower than the east side. 

The model-calculated position of the freshwater/saltwater 

interface in the three model cells representing the South Hollow 

well field is about 187 ft below NGVD 29 for current (2002) 

conditions (fig. 17), which is about 7 to 12 ft below the bottoms 

of the transition zones observed in monitoring wells SH-1 and 

SH-3. Although the model-calculated freshwater/saltwater 

interface position is slightly deeper than the field data indicate, 

the model-calculated change in this position with time (fig. 16) 

provides insight into how the freshwater/saltwater interface 

changes in response to pumping conditions. For example, at 

the South Hollow well field, the interface rises with time as 

pumping increases from 1955 to 1978 (fig. 16). In 1978, when 

pumping was reduced in response to the gasoline spill near the 

well field, the rise in the interface position stopped and the 

altitude of the interface began to decrease. By 1979, the 

pumping at the well field had stopped completely. The interface 

position did not return to its prepumping position, however, 

indicating a possible temporal lag in the reponse to the decrease 

in pumping (fig. 16). 

Other reasons for the lag in the response include that the 

pumping shortfall at South Hollow from 1978 to 1984 was 

compensated for by pumping at CCNS No. 4 (fig. 12) and

the NTAFB wells. Pumping from these wells may affect the 

position of the interface beneath South Hollow regionally—an 

effect that was not present in 1955. Another possible reason is 

an overall thinning of the Pamet flow lens because of the effects 

of sea level rise with time. 

Simulation results (fig. 16) also show that if the current 

pumping rate of 0.57 Mgal/d were maintained for 48 years, the 

interface position would rise by an additional 10 ft by year 

2050. These results indicate that the freshwater/saltwater 

interface has not yet reached equilibrium with respect to the 

current pumping. Alternatively, the fact that the modelcalculated freshwater/saltwater interface position has not 

reached equilibrium with respect to current pumping rates may 

be an artifact of the model representation of the aquifer system, 
20 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

20,000

0

5,000

10,000

15,000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS 

PER CENTIMETER AT 25 DEGREES CELSIUS

PUBLIC WATER-SUPPLY 

WITHDRAWALS, IN MILLIONS

OF GALLONS PER DAY

Well TSW-259 (screened at 95-100 feet below land surface)

Well TSW-260 (screened at 75-80 feet below land surface)

0.4

0

0.2

2001

FMAMJ J ASO N D

Figure 15. Specific conductance in monitoring wells TSW-259 and TSW-260 beneath Knowles Crossing well number 2 (KC2) and total pumping in 2001 at the Knowles Crossing well field, North Truro, Massachusetts.

such that the simulated hydraulic properties, model-boundary 

conditions, and (or) the simulation of average annual pumping 

and recharge conditions may result in a simulated response that 

does not represent the aquifer system accurately. Given the lack 

of long-term data on the previous position and movement of 

the interface, however, it is not possible to assess the modelsimulation accuracy with respect to future changes in the 

interface position. 

In addition to the effects of annual average pumping rates 

and constant recharge rates, the possible effects of seasonal 

changes in pumping and recharge on the position of the 

freshwater/saltwater interface in the vicinity of the three 

pumped well fields in the Pamet flow lens were simulated. The 

three well fields provide different pumping conditions for 

which a possible seasonal effect can be evaluated. The South 

Hollow well field is near the center of the flow lens, and has the 

highest pumping rates of the three well fields: 0.9 Mgal/d 

in-season (May–September) and 0.34 Mgal/d off-season 

(October–April). The Knowles Crossing well field is located 

nearest to the coast (about 1,000 ft from Cape Cod Bay) and has 

a pumping rate of 0.31 Mgal/d in-season and 0.12 Mgal/d offseason. The North Truro Air Force Base wells are near the 

Atlantic Ocean and are pumped only in-season at 0.33 Mgal/d. 

Pumping rates for the wells were determined on the basis of 

historical seasonal usage apportioned from the current average 

daily pumping rates of 0.57 Mgal/d for South Hollow, 

0.2 Mgal/d for Knowles Crossing, and 0.16 Mgal/d for the 

North Truro Air Force Base wells. 

The effects of 5 months of low recharge and high pumping 

from May through September (in-season) and 7 months of high 

recharge and low pumping from October through April (offseason) on the position of the freshwater/saltwater interface 

were simulated. The annual average recharge rate of 24 in/yr 

was apportioned into 18 in. for the 7-month off-season period 

and 6 in. for the 5-month in-season period. A detailed 

discussion of the seasonal-varying model simulations is 

provided in the appendix. 

The results indicate that although water levels changed 

in response to changing stress conditions, there was no 

appreciable change in the position of the freshwater/saltwater 

interface between in- and off-season-pumping and rechargerate changes. This result was the same for each of the three well 

fields despite the different hydrologic conditions at each of the 

sites. 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 21

-250

-230

-210

-190

-170

-150

-130

-110

-90

-70

-50

1940 1960 1980 2000 2020 2040 2060

0

0.8

0.9

0.7

0.6

0.5

0.4

0.3

0.2

0.1

FRESHWATER/SALTWATER INTERFACE

PUMPING RATE

YEAR

ALTITUDE, IN FEET BELOW NGVD 29

PUMPING RATE, IN MILLION GALLONS PER DAY

Figure 16. Model-calculated freshwater/saltwater interface and simulated pumping from 1955–

2050 at the South Hollow well field, North Truro, Massachusetts.

Generally, it is assumed that the potential for vertical 

movement of saltwater beneath the well fields would increase 

as the salt concentrations decrease near the top of the transition 

zone (Reilly and Goodman, 1985). Reilly and others (1987) 

observed annual changes in salt concentration from 3 to 10 

percent at a monitoring well beneath CCNS No. 4, a temporary 

supply well on the Cape Cod National Seashore that was 

pumped seasonally from 1978 to 1985 (fig. 12). 

Model-calculated salt concentrations indicated a transition 

zone that is much thicker than that observed in the aquifer 

(fig. 17). This discrepancy of model-calculated low saltwater 

concentration in areas known to be freshwater is caused by 

numerical dispersion, or model uncertainty, which is an artifact 

of the coarse grid and layer spacing used in the regional model. 

For the purpose of this investigation the model-calculated 50-

percent salt concentration (fig. 17) is assumed to be 

representative of the freshwater/saltwater interface. A more 

local-scale, subregional analysis would be required to minimize 

the effect of numerical dispersion and to simulate the actual 

distribution of salt concentrations in the aquifer explicitly for 

changing pumping and recharge conditions. 

The absence of an appreciable seasonal change in 

the freshwater/saltwater interface position calculated by 

the regional model was confirmed by borehole EM data 

collected in September 2000, January 2001, May 2001, and 

December 2001 at the well fields. The EM data (not shown) 

indicate that the freshwater/saltwater interface responds slowly 

to changing stress conditions and the time required to adjust to 

dynamic equilibrium is greater than the time interval of the 

seasonal cycle. 

A reduction in recharge was simulated for current 

pumping conditions to determine whether the freshwater/ 

saltwater interface position could be affected by a prolonged 

drought condition. For this analysis, the annual average 

recharge rate of 24 in/yr was reduced by 30 percent for a 3-year 

period. This analysis was performed at both the South Hollow 

and Knowles Crossing well fields. The objective of the 

simulation was to determine if the depth to the freshwater/ 

saltwater interface could be affected by the change in simulated 

recharge. 
22 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

50% 

75%

25%

50%

75% 

25%

50%

25%

75%

25%

0 100 200 300 400 500 600 700 800

0

0.0 0.5 1.0 1.5 2.0 2.5

0 100 200 300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5

0 100 200 300 400 500 600 700 800

0.0 0.5 1.0 2.0 2.5

SOUTH HOLLOW-1

(SH-1)

SOUTH HOLLOW-2

(SH-2)

SOUTH HOLLOW-3

(SH-3)

-300

-270

-240

-210

-180

-150

-120

-90

-60

-30

0

-300

-270

-240

-210

-180

-150

-120

-90

-60

-30

0

-300

-270

-240

-210

-180

-150

-120

-90

-60

-30

1.5

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

EM CONDUCTIVITY, IN MILLISIEMENS PER METER

ALTITUDE, IN FEET BELOW NGVD 29

MODEL-CALCULATED SALT CONCENTRATION

EM CONDUCTIVITY, MEASURED SEPTEMBER 2000

EXPLANATION

SALT CONCENTRATION, IN PERCENT

Figure 17. Profiles of electromagnetic (EM) logs measured in September 2000 and model-calculated changes in salt 

concentration for current (2002) conditions at the South Hollow well field, North Truro, Massachusetts.

The results of this simulation indicate that unlike water 

levels, the position of the interface responds much more slowly 

to the change in recharge; even with a prolonged drought 

condition, there was little effect on the simulated interface 

position at both well fields (results not shown). As is the 

case with seasonal fluctuations, it is possible that the zone of 

lower salt concentrations near the top of the transition zone 

and closer to the well screen may be affected more than the zone 

of 50-percent salt concentrations; however, this hypothesis 

could not be tested with the regional flow model because of 

the limitations imposed by the numerical dispersion that is 

associated with the dimensions of the model grid. 

Effects of Sea-Level Rise

Residents of coastal areas are becoming increasingly 

concerned about the effects of sea-level rise. The National 

Oceanographic Atmospheric and Administration (2003) 

reports a rising trend in sea level at the Boston Harbor Tidal 

Gage, which has been in operation since 1921, of about 

0.104 in/yr (2.65 ± 0.1 mm/yr) or about 0.87 ft/100 years. The 

Intergovernmental Panel on Climate Change (IPCC) predicts 

that global seas may rise by an additional 0.5 to 3.1 ft by 2100, 

with a best estimate of 1.6 ft (Intergovernmental Panel on 

Climate Change, 2001). This rate of rise would be nearly double 
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 23

the rate of rise observed at Boston Harbor over the past 80 

years. The primary concerns about the possible effects of sealevel rise include future higher rates of erosion than present, 

damage from higher storm-surge flooding (Theiler and 

Hammar-Klose, 2000), and landward intrusion of seawater in 

coastal marshes and wetlands (Donelly and Bertness, 2001). 

Rising sea levels also may affect coastal aquifers, such as 

those of Cape Cod (Intergovernmental Panel on Climate 

Change, 2001; Moore and others, 1997). Nuttle and Portnoy 

(1992) have speculated that increases in sea level may result in 

higher water levels in the tidally affected streams and wetlands 

of Lower Cape Cod, which could affect ground-water discharge 

to coastal areas. An analysis of the long-term change in water 

level at a USGS observation well, about 1,000 ft from Cape Cod 

Bay near the Knowles Crossing well field, indicates an increase 

of about 0.1 in/yr (2.1 mm/yr) from 1950–2001 (fig. 18) 

(McCobb and Weiskel, 2003). This water-level trend is 

consistent with the trend observed at Boston Harbor, which is 

about 50 mi northwest of North Truro (National Oceanographic 

Atmospheric and Administration, 2003).

Water Levels and Streamflows 

Analysis of the change in water levels at the long-term 

monitoring wells in the study area shows varying rates of 

change (fig. 19). In general, the rate of water-level rise appears 

to be greatest in the observation wells in the Pamet and Pilgrim 

flow lenses, near the coast and far from any non-tidal surfacewater bodies. Factors that could affect these trends, in addition 

to proximity to tidal waters, are proximity to pumping wells and 

the period of record over which the wells were measured. 

In response to these observed trends in sea-level rise and 

subsequent water-table rises, an annual increase of 0.104 in/yr 

(2.65 mm/yr) was simulated in the ground-water model from 

0.0 ft in 1929 to 0.63 ft in 2002. The model then was used to 

assess the effects of sea-level rise on the water-table 

configuration, coastal discharge, and the position and 

movement of the freshwater/saltwater interface. 

The results of the simulation compare favorably to data for 

two of the long-term ground-water observation sites in the 

Pamet (TSW-1) and Pilgrim (PZW-78) flow lenses (fig. 19). 

The results also indicate a water-table rise much less than the 

0.104 in/yr (2.65 mm/yr) at locations near non-tidal surfacewater bodies, such as throughout the Chequesset lens, which is 

dominated by the Herring River and numerous kettle-hole 

ponds. At the observation sites within the Chequesset flow lens 

(WNW-30, WNW-34, and WNW-108), the model-calculated 

changes are substantially less than those in the Pamet and 

Pilgrim flow lenses, but are greater than the observed negative 

changes (fig. 19). The difference between the simulated and 

observed levels may be the result of a complexity in the 

interactions between ground and surface waters not simulated 

in the flow model. Another example is in the Nauset flow 

lens at EGW-37, where the observed trend is 0.0043 in/yr 

(0.1 mm/yr) as compared to 0.022 in/yr (0.55 mm/yr) calculated 

by the model. This difference could be attributed, in part, to the 

artificial pond-level control at nearby Great Pond; this control is 

not represented in the ground-water model. 

The model-calculated changes in water-table altitude at 

TSW-179 and TSW-216 are less than the observed changes at 

these wells (fig. 19). These differences may be the result of tidal 

effects that extend farther inland at Pamet River and Mill Creek 

than the model-simulated tidal effects. 

Some other sites, such as WNW-17 and EGW-36 in the 

Nauset flow lens, show large declines in water-table altitudes 

with time rather than the rises calculated by the model. This 

discrepancy could be explained by local effects that are not 

represented in the flow model, such as ground-water pumping 

for water supply or irrigation at the National Park Service 

Headquarters near WNW-17 and the Nauset Regional High 

School near EGW-36. 

The general trend in both the simulated and observed 

water-table altitude is that, for the most part, water levels are 

increasing with time in response to rising sea level, and the 

magnitude of the increase appears to be affected by the 

proximity to non-tidal surface-water bodies. The flow model 

was used to assess the regional effects of simulated sea-level 

rise and the local effects at or near pumping wells. ALTITUDE OF WATER LEVEL, IN FEET ABOVE NGVD 29

0.1 inch/year average 

water-level rise

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

1950 1960 1970 1980 1990 2000

Mean = 2.53 feet

Highest = 4.02 feet (March 23, 1983)

Lowest = 1.20 feet (September 11, 1954)

Figure 18. Water-table altitude at observation well TSW-1, 

North Truro, Massachusetts, 1950–2002 (from McCobb and 

Weiskel, 2003).
24 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Chequesset 

Neck

Pamet River

Herring Riv er

Mill Creek

Great

Pond

70o15' 70o00'

42o

00'

41o

52'30"

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

0 5 MILES

0 5 KILOMETERS

TSW-1

TSW-89

TSW-92

TSW-179

TSW-216

EGW-36

EGW-37

WNW-17

WNW-34

WNW-108

TSW-203

TSW-200

WNW-30

PZW-78

Site X

Site Y

EXPLANATION

CAPE COD NATIONAL SEASHORE

PONDS, MARSHES, AND WETLANDS

RIVERS

FLOW-LENS BOUNDARY

PUBLIC-SUPPLY WELL

OBSERVATION WELL AND IDENTIFIER EGW-36

Pilgrim flow lens

Pamet 

flow lens

Chequesset 

flow lens

Nauset 

flow

lens

Little Pamet R

Simulated 

EGW-36 -15.1 1.7 1975–2002

EGW-37 .1 .6 1975–2002

WNW-17 -5.4 .9 1962–2002

WNW-30 -.9 .4 1975–2002

WNW-34 -1.2 .03 1975–2002

WNW-108 -2.1 .9 1979–2002

TSW-216 2.1 .9 1975–2002

TSW-1 2.1 2.4 1950–2002

TSW-89 .2 1.2 1957–2002

TSW-92 -.7 1.7 1972–2002

TSW-179 3.1 .1 1973–2002

TSW-203 -2.4 1.7 1973–2002

PZW-78 2.9 2.5 1975–2002

Rate of Change (Millimeters/Year) Period of

Record

(Year)

Well

Identifier Measured 

ALTITUDE OF WATER TABLE

Nauset Chequesset Pamet Pilgrim

Figure 19. Locations of long-term observation wells and the measured and model-calculated increase in the altitude of the 

water table with time, Lower Cape Cod, Massachusetts.
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 25

The regional effects of sea-level rise on the Pamet flow 

lens were assessed by examining the model-calculated change 

in water levels and position of the freshwater/saltwater interface 

at two locations from 1929 to 2002 (current) and from 2002 to 

2050. Site X (fig. 19) is about 3,000 ft from Cape Cod Bay and 

adjacent to the Little Pamet River. Site Y is about 3,000 ft from 

the Atlantic Ocean, near well TSW-203 (fig. 19), and far from 

any non-tidal surface-water bodies. The model-calculated depth 

to the freshwater/saltwater interface and the simulated 

subsurface lithology are similar beneath both sites. 

Results of the model simulations indicate that from 1929 

to 2002 the water-table altitude at site X increased by 0.15 ft 

(fig. 20A). This increase indicates that, at this location (site X), 

the water table only has risen by about 0.02 in/yr (0.62 mm/yr), 

or about 24 percent of the 0.63 ft of regional sea-level rise. At 

site Y the model-calculated increase of 0.43 ft yields a watertable rise of about 0.07 in/yr (1.78 mm/yr), or about 68 percent 

of the 0.63 ft of regional sea-level rise (fig. 20A). Because the 

local sea-level position is simulated to be 0.63 ft higher in 2002 

than it was in 1929, the increase in water level at site X of 

0.15 ft relative to NGVD 29 actually is a net decline of about 

0.48 ft relative to the increased sea level (fig. 20B). The increase 

in water level at site Y of 0.43 ft relative to NGVD 29 actually 

is a net decline of about 0.20 ft relative to the increased sea level 

(fig. 20B). 

The decline in the water-table altitude relative to local sea 

level can be explained, in part, by the increase in ground-water 

pumping in the Pamet flow lens from 1929 to 2002. During 

this time, ground-water pumping increased from 0.30 to 

0.94 Mgal/d. Nearly all of the water pumped from the Pamet 

flow lens is exported to Provincetown in the Pilgrim flow lens. 

This export of water creates the potential for an overall decline 

in water levels with time. If this decline were caused only by the 

loss of pumped water from the flow lens, then the decrease in 

water levels would be largest near the pumped wells in the 

center of the flow lens, and far from the coast or discharge areas, 

such as the Little Pamet River. Barlow and Hess (1993) showed 

that water-level declines in response to pumping near the 

Quashnet River in western Cape Cod were dampened by 

the interaction between ground and surface waters. If this 

dampening effect was the case in the Pamet flow lens, then the 

net loss at site Y, which is near a pumped well and away from 

surface-water discharge areas, should be greater than site X, 

which is near the Little Pamet River; however, the simulated net 

water-level decline at site X is more than double the net waterlevel decline at site Y. The results indicate that the difference in 

water-level declines at these sites may be a result of changing 

sea level rather than ground-water pumping. 

The surface-water bodies that dampen the decline 

in water levels in response to increased pumping also may 

dampen the rise in water levels in response to sea-level rise. 

Ground-water-fed, or gaining streams prevent the surrounding 

water table from rising appreciably above the altitude of the 

streambed. As the water table rises in response to sea-level rise, 

the amount of ground-water discharge to the stream increases 

because the increased height of the water table adjacent to the 

stream results in increased streamflow rather than a higher 

water-table altitude at the stream. 

In the Pamet flow lens, the model-calculated ground-water 

discharge to the Little Pamet River increased from 0.39 to 

0.62 ft3/s in response to sea-level rise from 1929 to 2002. The 

increase in streamflow in the Little Pamet River over this period 

is related directly to the increased water-table altitude in the 

vicinity of the river. The surface-water bodies, such as the Little 

Pamet River, are analogous to pumping wells in the way in 

which they remove freshwater from the aquifer. The "pumping 

rates" of the streams depend upon the magnitude of water-table 

rise in response to the sea-level change. This "pumped" water 

leaves the aquifer as streamflow to the coast. The water levels 

have risen by a greater rate at site Y than site X probably 

because of the proximity of site X to the Little Pamet River. 

The net decline in the water-table altitude at site Y relative 

to the rise in local sea level may be the result of exported 

pumped water from the Pamet flow lens lowering regional 

water levels. However, the total simulated discharge to nontidal surface-water bodies in the flow lens increased by about 25 

percent from 1929 to 2002. Therefore, this net decline also may 

be the result of the regional effect of streams and wetlands on 

the ground-water levels throughout the Pamet flow lens. 

The effect of increased ground-water discharge to tidally 

affected surface-water bodies is negligible, especially by 

comparison to the effect of non-tidal surface-water bodies;

it is likely that the stream stage in tidal waters will rise in 

conjunction with sea level at a rate equal or greater than the 

rise in water levels in the aquifer. Nuttle and Portnoy (1992) 

hypothesize that this rise in stream stage may result in an overall 

decrease in ground-water discharge to coastal surface-water 

bodies with time.

Freshwater/Saltwater Interface

At sites X and Y (fig. 19), the water-level altitudes 

decreased relative to sea level in response to the rising sea level 

(fig. 20). The depth to the freshwater/saltwater interface 

decreased, or alternatively, the altitude of the freshwater/ 

saltwater interface increased relative to sea level. The 

difference in the model-calculated change in altitude of the 

freshwater/saltwater interface between sites X and Y is 

controlled by the difference between the sites in the net decline 

in water levels relative to local sea level. 
26 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

SEA-LEVEL RISE 

PER YEAR

Millimeters

6.0

2.65

Inches

0.236

0.104

0

1

2

3

4

5

6

1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

WATER-TABLE ALTITUDE ABOVE NGVD 29, IN FEET WATER-TABLE ALTITUDE ABOVE LOCAL SEA LEVEL, IN FEET

0

1

2

3

4

5

6

7

YEARS

A.

B.

Site X

Site X

Site Y

Site Y

Sea level

Figure 20. Model-calculated water-table altitude from 1929 to 2050 at sites X and Y and simulated 

changes in sea level A, above NGVD 29; and B, above local sea level, North Truro, Massachusetts.
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 27

The flow model was used to analyze the position and 

movement of the freshwater/saltwater interface beneath 

sites X and Y to determine the response of the interface to the 

rise in sea level from 0.0 ft in 1929 (NGVD 29) to its presentday (2002) altitude of 0.63 ft (NGVD 29). The position of the 

interface beneath site X in 1929 was calculated to be 215 ft 

below NGVD 29 (fig. 21). In response to sea-level rise and 

increased ground-water pumping, the altitude of the interface 

increased by about 0.37 ft by 2002. A similar increase was 

observed for site Y, where the altitude of the interface was 

calculated to be about 246 below NGVD 29 and rose by about 

0.37 ft from 1929 to 2002 (fig. 21). 

The lack of movement of the freshwater/saltwater 

interface with time in response to the change in sea level may 

be the result of a slow response of the interface to changing 

stress conditions. To determine whether the interface position is 

affected by the sea-level change and whether there is a lag in the 

response, three simulations were made: (1) the rate of sea-level 

rise was assumed to be zero for the next 48 years from 2002 to 

2050, (2) sea level was assumed to continue to rise at 0.104 in/yr 

(2.65 mm/yr) from 2002 to 2050, and (3) the rate of sea-level 

rise was assumed to increase to 0.236 in/yr (6 mm/yr) from 

2002 to 2050, as hypothesized by Donnelly and Bertness 

(2001). 

In the first simulation described above, sea level remained 

at a constant altitude of 0.63 ft above NGVD 29 from 2002 to 

2050 and the amount of ground-water pumping remained at the 

2002 pumping rate. The water-table altitude at both sites X and 

Y remained constant; however, the altitude of the freshwater/ 

saltwater interface did change beneath these sites. The altitude 

of the freshwater/saltwater interface increased by 1 ft beneath 

site Y, whereas at site X, the altitude of the interface increased 

by about 7 ft. The change in the position of the interface at these 

sites indicates that there is a lag in the reponse of the interface 

and that the proximity of site X to the Little Pamet River may 

result in more of a change in the interface position than that 

calculated for site Y. 

In the second simulation, sea level continued to rise at the 

rate of 0.104 in/yr (2.65 mm/yr) from 2002 to 2050. In response 

to this continued sea-level rise of 0.42 ft (for a cumulative sealevel rise of 1.05 ft), the altitude of the water table at sites X and 

Y declined by a net of 0.34 and 0.14 ft, respectively. By 2050, 

the water-table altitude at site X was 3.41 ft above the local sealevel altitude of +1.05 ft NGVD 29, in comparison to an altitude 

of 3.75 ft above the local sea-level altitude of +0.63 NGVD 29 

in 2002 (fig. 20B). At site Y the water-table altitude in 2050 was 

4.61 ft above the local sea-level altitude of +1.05 ft NGVD 29 

in comparison to 4.75 ft above the local sea-level altitude of 

+0.63 ft NGVD 29 in 2002 (fig. 20B). 

SEA-LEVEL RISE 

PER YEAR

Millimeters

6.0

2.65

0

Inches

0.236

0.104

0

-210

-200

-190

-240

-230

-220

-250

1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

ALTITUDE OF FRESHWATER/SALTWATER INTERFACE

 BELOW NGVD 29, IN FEET

YEARS

Site X

Site Y

Figure 21. Model-calculated altitude from 1929 to 2050 of the freshwater/saltwater interface relative to 

NGVD 29 beneath sites X and Y, North Truro, Massachusetts.
28 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

During the period of 1929 to 2050, the rises in water-table 

altitudes of 0.23 ft and 0.71 ft at sites X and Y, respectively, 

are less than the simulated rise in sea level of 1.05 ft for that 

same period (fig. 20A). As a result, the net change in water-table 

altitudes at these wells relative to sea level would be about 

-0.82 ft for site X and -0.34 ft for site Y (fig. 20B). During this 

same period, however, the streamflow in the Little Pamet River 

increased two-fold from 0.39 ft3/s in 1929 to 0.78 ft3/s in 2050. 

It is this increase in streamflow that may account for the 

difference in water-level changes between sites X and Y. 

The altitude of the freshwater/saltwater interface also 

changed beneath the sites as sea level continued to rise from 

2002 to 2050. The altitude of the interface increased by 12 ft 

beneath site X and 2 ft beneath site Y relative to their 2002 

positions. Because the sea level rose from about 0.63 to 1.05 ft 

above NGVD 29 over this period, the net change in altitude of 

the interface relative to sea level in 2050 beneath sites X and Y 

is 11.58 ft and 1.58 ft, respectively. 

The increase in the altitude of the freshwater/saltwater 

interface beneath site X changed at a much greater rate from 

2002 to 2050 than the rate from 1929 to 2002. From 1929 to 

2002, the altitude of the interface increased by only 0.37 ft or at 

rate of 0.06 in/yr (1.50 mm/yr). From 2002 to 2050, the altitude 

of the interface increased by another 11.58 ft in only 48 years or 

at a rate of 2.90 in/yr (73.53 mm/yr). Of this model-calculated 

11.58 ft, an increase about 7 ft can be attributed to the lag in the 

response to the sea-level rise from 1929 to 2002. The additional 

4.58 ft of increase is a result of the continued sea-level rise 

of 0.104 in/yr (2.65 mm/yr) from 2002 to 2050. This rate of 

rise from 2002 to 2050 is about 1.15 in/yr (29.08 mm/yr) 

and is substantially greater than the rate of rise of 0.06 in/yr 

(1.50 mm/yr) from 1929 to 2002. The total increase in the 

altitude of the interface from 1929 to 2050 beneath site X, 

however, is about 11.95 ft, or 1.19 in/yr (30.10 mm/yr) in 

response to a decline in water-table altitude of 0.82 ft over the 

same period. 

After accounting for the model-calculated 1 ft of rise that 

can be attributed to the lag in the response to the sea-level rise 

from 1929 to 2002 beneath site Y, the 0.58-ft increase in the 

altitude of the freshwater/saltwater interface from 2002 to 2050 

is substantially less than that calculated for site X, but still is 

greater than the increase in the interface altitude of 0.37 ft for 

this location from 1929 to 2002. On the basis of these results 

and their uncertainty, however, it would appear that the position 

of the interface beneath this site is not substantially affected by 

the change in sea-level altitude over time. 

In the third simulation, the potential effects of an increased 

rate in sea-level rise from 0.104 in/yr (2.65 mm/yr) to 

0.236 in/yr (6.0 mm/yr) were assessed, as hypothesized by 

Donnelly and Bertness (2001). In response to this rise in the sealevel altitude from about 0.63 ft above NGVD 29 in 2002 to 

about 1.58 ft above NGVD 29 in 2050, the water-table altitude 

declined by about 0.77 ft at site X and about 0.25 ft at site Y 

relative to local sea level (fig. 20A) and the streamflow in the 

Little Pamet River increased to about 1.2 ft3/s, nearly twice the 

flow in 2002. 

In response to this additional increase in the rate of sealevel rise, the altitude of the freshwater/saltwater interface 

between 2002 and 2050 increased by 16.05 ft beneath site X 

and 2.05 ft beneath site Y relative to local sea level in 2050 

(fig. 20B). Of this 16.05 ft of increase beneath site X, about 7 ft 

can be attributed to the lag in the response to the sea-level rise 

from 1929 to 2002. Therefore, the total increase in the altitude 

of the interface is about 9.05 ft for a sea-level rise of 0.236 in/yr 

(6.0 mm/yr) compared to the calculated increase in interface 

altitude of 4.58 ft for a sea-level rise of 0.104 (2.65 mm/yr) for 

the same period. Of the 2.05 ft of increase beneath site Y, about 

1 ft can be attributed to the delay in the response to the sea-level 

rise from 1929 to 2002. Therefore, the total increase in the 

altitude of the interface is about 1.05 ft for a sea-level rise of 

0.236 in/yr (6.0 mm/yr) compared to the calculated increase in 

altitude of 0.58 ft for a sea-level rise of 0.104 in/yr (2.65 mm/yr) 

for the same period (fig. 20B). 

Based on the analysis described above, sea-level rise 

over time results in a thinning of the aquifer in the Pamet flow 

lens. This thinning results because the rate at which the water 

table increases is less than the rate at which sea level rises 

resulting in a net decline in the water-table altitude, which 

results in an increase in the altitude of the freshwater/saltwater 

interface. The areas where the effect on the position of the 

freshwater/saltwater interface is greatest are those areas near 

non-tidal streams and wetlands. The effect is substantially less 

where the water-table rise is not limited by non-tidal streams 

and wetlands. The rate of rise of the freshwater/saltwater 

interface is affected by a lag in the response of the interface to 

changes in water-table altitude. 

The effect of sea-level rise was analyzed in the other flow 

lenses for the same period and a similar effect of increased 

water levels and streamflows was observed; however, the effect 

on the freshwater/saltwater interface was negligible during 

this period. In the Pilgrim flow lens, non-tidal surface-water 

discharge is negligible and the water level at PZW-78 increased 

by about the same rate as sea level (fig. 19). Because the water 

levels in this flow lens rise in conjunction with sea level, the 

difference between the water table and sea level does not 

change appreciably and, therefore, the position of the 

freshwater/saltwater interface remains static. 

In the Chequesset flow lens, and to a lesser extent in the 

Nauset flow lens, there are a substantial number of nontidal 

surface-water discharge areas, and the rate of water-level rise 

in these flow lenses is much less than the rate of simulated 

sea-level rise (fig. 19). Although the model-calculated rate of 

water-level increases in these flow lenses are substantially 

lower than the rate of sea-level rise, there is no appreciable 

change in the model-calculated interface position beneath these 

flow lenses as was calculated beneath the Pamet flow lens. 

A possible reason for this difference in the response of the 

interface could be differences in subsurface lithology simulated 

in the flow lenses. It is these differences that may result in much 

slower response times of the freshwater/saltwater interface in 

the Chequesset and Nauset flow lenses than were calculated in 

the Pamet flow lens.
Simulated Interaction Between Freshwater- and Saltwater-Flow Systems 29

Pumping Wells 

Another hydrologic stress that affects the height of the 

water table, and, therefore, can be affected by the rise in sea 

level, is pumping wells. The South Hollow well field in the 

Pamet flow lens has been in operation since 1955 and currently 

provides, on average, over 60 percent of the nearly 1.0 Mgal/d 

of water pumped from the Pamet lens for public supply. The 

model-calculated altitude of the freshwater/saltwater interface 

beneath the center of this well field is about 187 ft below NGVD 

29 for 2002, which is 43 ft higher than the altitude calculated for 

1954 before pumping started (fig. 16). 

In the first simulation, sea level was held constant at the 

2002 altitude of about 0.63 ft above NGVD 29 and pumping 

rates were maintained at current (2002) rates from 2002 to 

2050. The altitude of the freshwater/saltwater interface 

increased by about 10 ft beneath the South Hollow well field 

from 2002 to 2050. This 10-ft increase is much greater than the 

1-ft increase calculated at site Y and is slightly greater than the 

7-ft increase calculated at site X. The model-calculated increase 

in the altitude of the freshwater/saltwater interface beneath the 

well field from 2002 to 2050 is a delayed response to the 

changes in pumping from 1955 to 2002, a result of the rising sea 

level from 1929 to 2002, or a combined effect of both of these 

factors. 

In the second simulation, sea level was set to rise at 

0.104 in/yr (2.65 mm/yr) and pumping rates were held constant 

at 2002 rates. The results from this simulation indicate that by 

2050 the altitude of the interface would increase by 13 ft from 

the 2002 position, 3 ft more than was calculated when sea level 

was set at the 2002 altitude of 0.63 ft above NGVD 29. This 

increase in the altitude of the interface is similar to the increase 

calculated beneath site X. 

In the third simulation, the rate of sea-level rise was set to 

increase from 0.104 in/yr (2.65 mm/yr) to 0.236 in/yr (6 mm/yr) 

and pumping was held constant at 2002 rates. The altitude of the 

interface beneath the well field increased by 15 ft between 2002 

and 2050, 2 ft more than for the rise of 0.104 in/yr (2.65 mm/yr), 

but less than was seen at site X. The reason for this difference 

in responses at the well field and site X can be attributed to the 

fact that the pumping rate is held constant at the well field. In 

the case of site X, where the ground-water discharge to the 

stream continued to increase in response to the rising water 

table, the water-table rise in the vicinity of the stream was 

limited. This limited rise created a greater decline in water 

levels relative to sea level near the stream than the declines 

observed at areas farther away from the stream. 

With the pumping rates held constant for the 48-year 

simulation, the water level at the well field rose by 0.19 ft. This 

rise represents a decline with respect to the new sea-level 

position of 1.05 ft above NGVD 29 in 2050 calculated for a rate 

of sea-level rise of 0.104 in/yr (2.65 mm/yr). The net decline of 

0.23 ft in the water-table altitude at the well field, the difference 

between simulated rise in sea level of 0.42 ft from 2002 to 2050, 

and the 0.19 ft rise in the altitude of the water table, is less than 

the net decline of 0.34 ft at site X near the Little Pamet River. 

The constant pumping rate does not affect the response of the 

water table to sea-level rise, whereas in the vicinity of the 

stream at site X, the rise in the water table is offset by the 

increase in streamflow. Thus, the rate of rise at the water table 

decreases and causes the interface to rise. 

If the pumping rate at the South Hollow well field were to 

increase enough for the water-table altitude near the well field 

to remain constant even though sea level is rising, then the 

interface would be expected to rise at a similar rate as that 

observed at site X near the stream. If the pumping rate were 

increased enough for the water-table altitude at the well field to 

decline while the surrounding water table rose because of sealevel rise, then the altitude of the interface would be affected by 

both increased pumping and sea-level rise. 

Sea-level rise and increased pumping rates have a similar 

effect on the relation between the yield of pumping wells 

and the position of the freshwater/saltwater interface. This 

relation may necessitate limiting pumping rates in response to 

sea-level rise in order to protect public-water-supply systems 

from saltwater contamination. Past studies to evaluate the 

long-term safe yield at the South Hollow well field have 

determined that there is the potential for saltwater intrusion if 

the interface rises by more than 33 percent of the distance from 

the predevelopment interface position to the bottom of the well 

screen (Camp Dresser & McKee, Inc., 1985; Environmental 

Partners Group, Inc., 2002). In the case of the South Hollow 

well field, a 33-percent change in the interface position would 

be a rise of 63 ft from a model-calculated predevelopment 

altitude of 230 ft below NGVD 29. 

The model-calculated altitude of the interface beneath 

the South Hollow well field for 2002 is 43 ft above the 

predevelopment position (fig. 16) or a 23-percent change in the 

distance from the predevelopment interface position to the 

simulated altitude of the bottom of the well screen of -40 ft 

NGVD 29. The model simulations indicate that, during the next 

48 years, the altitude of the interface may increase by as much 

as an additional 15 ft, depending on the rate of sea-level rise, 

even if pumping were held constant at 2002 rates. This 

additional 15-ft of rise would result in a total decrease in the 

depth of the interface position of 58 ft by 2050, or a 31-percent 

change in the distance from the predevelopment interface 

position to the bottom of the well screen. 

On the basis of the analysis described above, the interface 

beneath the South Hollow well field in 2002 has risen only 

by 23 percent of the distance between the predevelopment 

interface position and the bottom of the well screen, which is 

much less than the 33-percent change threshold for safe yield. 

However, if the current (2002) pumping rates are held constant 

for the next 48 years, this percentage could increase to 31 

percent. Presumably, if the pumping rates were increased from 

the current rate during this period, the percentage change could 

exceed the 33-percent threshold for safe yield.
30 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Simulation of Proposed 

Ground-Water-Pumping Scenarios

The flow model used in this investigation was developed 

to serve as a tool to assist the National Park Service and the 

communities of Lower Cape Cod in managing their water 

resources so that the future demand for water supply can be met 

without adversely affecting the ponds, streams, wetlands, and 

coastal areas throughout Lower Cape Cod. Simulations were 

made to evaluate whether future pumping could affect surfacewater bodies, such as streams and ponds, and the position and 

movement of the freshwater/saltwater interface. Examples were 

selected to demonstrate how the flow model developed for this 

investigation could be used to determine the aquifer-system 

response to changes in pumping and recharge conditions. The 

proposed pumping well sites, pumping rates, and wastewater 

return locations simulated in the model were provided by the 

four towns of Lower Cape Cod through the stakeholders 

committee set up for this project. 

Effects on Streamflow

The residences of the town of Eastham in the Nauset flow 

lens (fig. 1) currently obtain nearly all of their water supply 

from small-capacity private drinking-water wells. Water 

pumped from these sites is returned to the aquifer by on-site 

private septic systems. Concerns over possible sources of 

contamination such as residential septic systems, the town 

landfill, and the commercial areas along Route 6, have 

prompted the Eastham Water Resources Advisory Board 

(WRAB) to consider two sites for public-supply wells to 

provide, on average, 1.10 Mgal/d of water (Dr. Karl Weiss, 

Eastham Water Resources Advisory Board, written commun., 

2003). 

The WRAB currently is considering two potential sites, 

the Roach site and Water District G site (fig. 12), to provide a 

total of 1.10 Mgal/d of water to meet the future water demand 

for the northern part of town. Given the locations of these sites, 

the hydrogeology of the Nauset flow lens, and the depth to the 

freshwater/saltwater interface, the greatest concern is the effect 

of pumping on Hatches Creek, which is in the northwestern part 

of town (fig. 12). 

Hatches Creek is a small freshwater stream that flows from 

east to west into Sunken Meadow, a saltwater marsh along the 

shore of Cape Cod Bay. The model-calculated discharge to this 

stream under current (2002) conditions is about 0.52 ft3/s at 

West Road (fig. 12). The effect of pumping on streamflow in 

Hatches Creek at West Road was calculated for three different 

pumping scenarios at the proposed well sites. 

First, a simulation was made with each well pumping 

at 0.55 Mgal/d for a total of 1.1 Mgal/d. It was assumed that 

85 percent of this water was returned to the aquifer as 

wastewater within the areas designated as residential land use 

(Massachusetts Executive Office of Environmental Affairs–

Community Preservation Initiative, 1999). These areas 

correspond with the Traffic Analysis Zones (TAZ), which 

partition the town into eight separate zones (fig. 12) (Landuse 

Collaborative, 1996), identified by the WRAB as areas likely to 

receive future public supply. Based on these pumping and 

wastewater-disposal rates, the streamflow in Hatches Creek 

would be expected to decrease from 0.52 to 0.15 ft3/s, or about 

71 percent of the total pre-pumping flow. 

The reduction of streamflow described above is illustrated 

schematically in figure 22. If wells are pumped upgradient of a 

surface-water body, such as Hatches Creek, the pumping well 

can capture water that otherwise would discharge to the surfacewater body. Ground-water discharge to Hatches Creek is 

reduced, thereby reducing streamflow. If the pumping from 

wells upgradient of discharge areas becomes large enough, 

ground-water flow directions can be reversed and water can be 

drawn directly from surface-water bodies to the pumped well 

(fig. 22C). Streamflows and pond levels then are further 

reduced. 

Additional simulations were made with each well pumping 

separately to determine the effects on streamflow in Hatches 

Creek to changes in pumping conditions. When pumping was 

simulated only at the Roach site at a rate of 0.55 Mgal/d, the 

streamflow was reduced by about 0.23 ft3/s to 0.29 ft3/s, 

or 56 percent of the total pre-pumping flow. When pumping 

was simulated only at the Water District G site at a rate of 

0.55 Mgal/d, the streamflow was reduced by about 0.07 ft3/s 

to 0.45 ft3/s, or 87 percent of the total pre-pumping flow. 

The effect of streamflow reduction from both wells pumped 

separately is 0.30 ft3/s (0.23 + 0.07 ft3/s), which is 0.07 ft3/s less 

than the 0.37 ft3/s of reduction that was calculated when both 

wells were pumped simultaneously. The difference in the effect 

on streamflow from these simulations can be attributed to the 

difference in the source of water to the wells and the stream for 

the different pumping scenarios (figs. 23A–D). 

Under pre-pumping conditions, the water discharging to 

Hatches Creek (0.52 ft3/s) is derived from the area shown in 

figure 23A. When only the Roach site is pumped at 0.55 Mgal/d, 

the area contributing recharge to this well shows that the well 

captures water that otherwise would have discharged to Hatches 

Creek (fig. 23B). When only the Water District G site is 

pumped, most of the water discharging at that well does not 

derive from the area that contributes water to Hatches Creek; 

therefore, there is less effect from pumping at this site on 

Hatches Creek (fig. 23C). When both wells are pumping at 

0.55 Mgal/d, the area contributing recharge to the Roach site is 

affected by the pumping at Water District G (fig. 23D). In 

response to the pumping at the Water District G site, the area 

contributing recharge to the Roach site shifts to an area that 

captures more water that would have otherwise discharged to 

Hatches Creek (fig. 23D). This shift may explain why the 

streamflow reduction is greater with both wells pumping than 

the arithmetic sum of the separate reductions caused by each 

well. 
Simulation of Proposed Ground-Water-Pumping Scenarios 31

Schematic diagram, not to scale

Unconfined aquifer

Unconfined aquifer

Q1

Q2

Recharge area

Land surface

Water table

C.

B.

A.

Land surface

Land surface

Surface-waterbody

Surface-waterbody

Surface-waterbody

Unconfined aquifer

Water table

Water table

Bedrock

Bedrock

Bedrock

Recharge area

Recharge area

Figure 22. Schematic diagram of a hypothetical aquifer showing ground-water 

discharge to a surface-water body with A, no pumping; B, pumping at a rate such 

that the well would capture water that would otherwise discharge to the surfacewater body; and C, pumping at a higher rate so that the flow direction is reversed 

and the well pumps water from the surface-water body (modified from Alley and 

others, 1999).

A final simulation was made to determine the effect 

of pumping from Water District G site alone at a rate of 

1.10 Mgal/d on Hatches Creek. The area contributing recharge 

to the well at this pumping rate is double the size of the 

area contributing recharge to the well for the lower rate of 

0.55 Mgal/d, and as a result the simulated well captures more 

flow that would have otherwise discharged to Hatches Creek 

(fig. 24). The streamflow reduction of 0.15 ft3/s (29 percent of 

pre-pumping flow) from the higher pumping rate at the Water 

District G site, however, is lower than the 0.23 ft3/s (56 percent 

of pre-pumping flow) from the Roach site pumping alone at 

0.55 Mgal/d. 

Effects on Water Levels in Kettle-Hole Ponds

The residents of the town of Wellfleet receive all of their 

drinking water from the Chequesset and Nauset flow lenses. As 

is the case in Eastham, nearly all of Wellfleet’s residents obtain 

their drinking water from small-capacity domestic-supply wells 

and the wastewater is returned to the aquifer through on-site 

private septic systems. The only public supply currently 

operating in Wellfleet is a small system that provides about 

7,000 gal/d of water to the residences of Coles Neck in the 

vicinity of the town landfill. 
32 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

4

2

16

12

16

14

10

8

6

4

2

70o00'

41o

52'30"

41o

51'

69o

57'30"

West

Rd

10

2

16

14

12

10

8

6

4

70o00’ 69o

57’30”

West

Rd

2

16

14

12

10

8

6

4

70o00’

41o

52’30”

41o

51’

69o

57’30”

West

Rd

2

16

14

12

10

8

6

4

70o00’ 69o

57’30”

West

Rd

A.

C.

B.

D.

0 2 MILES

0 2 KILOMETERS

2

EXPLANATION

Figure 23. Location of model-calculated contributing areas to A, Hatches Creek for current (2002) conditions; B, Hatches Creek and 

the Roach site pumping at 0.55 million gallons per day; C, Hatches Creek and Water District G site pumping at 0.55 million gallons per 

day; and D, Hatches Creek, Water District G site, and the Roach site each pumping at 0.55 million gallons per day, Eastham, 

Massachusetts. 
Simulation of Proposed Ground-Water-Pumping Scenarios 33

4

2

16

14

12

10

8

6

16

14

12

10

8

6

4

2

0

0 500 METERS

3,000 FEET

70o00'

41o

52'30"

41o

51'

69o

57'30"

West Road

CAPE COD NATIONAL SEASHORE

PONDS, MARSHES, AND WETLANDS

CONTRIBUTING AREA TO HATCHES CREEK 

FOR 1.10 MILLION GALLONS PER DAY 

PUMPING RATE

CONTRIBUTING AREA TO WATER DISTRICT 

G SITE FOR 0.55 MILLION GALLONS PER 

DAY PUMPING RATE

ADDITIONAL CONTRIBUTING AREA TO WATER 

DISTRICT G SITE FOR 1.10 MILLION GALLONS 

PER DAY PUMPING RATE 

ADDITIONAL CONTRIBUTING AREA TO 

HATCHES CREEK FOR 0.55 MILLION GALLONS 

PER DAY PUMPING RATE

LOCATION OF PROPOSED WATER DISTRICT

G SITE

EXPLANATION

Figure 24. Location of model-calculated contributing areas to Hatches Creek and the proposed Water District G well 

pumping at 0.55 and 1.10 million gallons per day, Eastham, Massachusetts.
34 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

The town recently has begun contruction (September 

2003) to expand the existing public-supply system to 

the municipal buildings and residences in the downtown 

area (TAZs 12 and 13, fig. 12) at an average daily rate of 

0.30 Mgal/d. This proposed increase in public supply would 

be obtained from increased pumping at the Coles Neck site and 

the possible addition of two new well sites, the Boy Scout Camp 

and Wellfleet By the Sea sites (fig. 12). The town currently is 

conducting hydrologic investigations to determine if these three 

sites can be pumped at 0.1 Mgal/d each. 

A simulation was made with each well pumping at a rate 

of 0.1 Mgal/d and about 0.255 Mgal/d of wastewater returned to 

the aquifer at the areas designated as residential and municipal 

land uses in TAZs 12 and 13 (fig. 12) (Massachusetts Executive 

Office of Environmental Affairs–Community Preservation 

Initiative, 1999). Because the potential effects of pumping at 

these sites on pond levels in Duck, Dyer, Long and Great Ponds 

(figs. 12, 25) is of great concern, determining this effect was 

a principal goal of this analysis. Of this group of kettle-hole 

ponds in the vicinity of the proposed pumping wells, Duck Pond 

is most likely to be affected by possible future pumping because 

of its proximity to the top of the water-table mound and to the 

proposed pumping wells. 

The potential effect of pumping on Duck Pond differs 

from the effect of pumping on surface-water bodies shown 

schematically in figure 22 and on Hatches Creek (figs. 23 and 

24), because Duck Pond is upgradient instead of downgradient 

of the proposed pumping wells. As a result, the simulated 

pumping wells cannot capture water that otherwise would have 

discharged to the pond. An analysis of the contributing areas to 

Duck Pond and the three pumping wells indicates that the wells 

do not capture any water that otherwise would discharge to the 

pond under current conditions (fig. 25B). 

Although the proposed wells do not capture water that 

otherwise would have discharged to Duck Pond, the resulting 

decline in ground-water levels in response to pumping does 

extend to the pond. For average annual pumping conditions, 

the pond level would be lowered by nearly 0.4 ft (fig. 25B). 

Simulations were made to determine if the location of the pond 

near to the top of water-table mound may account for the 

seemingly large effect on the pond level by a small increase in 

pumping. 

Previous studies on Cape Cod have shown that the natural 

fluctuation in water levels in response to changes in recharge 

generally increases from the coast to the top of the water-table 

mounds (Letty, 1984; Masterson and Barlow, 1996). In 

addition, streams also appear to limit the fluctuation of the 

water table in the surrounding area. Duck Pond, which is near 

the top of the southern water-table mound in the Chequesset 

flow lens (fig. 1), has a reported maximum annual fluctuation of 

about 1.6 ft determined from monthly measurements from 1994 

to 1999 (Sobczak and others, 2003). In contrast, the maximum 

annual fluctuation at Gull Pond, which is close to the Herring 

River and connected to the nearby kettle ponds by a network of 

shallow, dug channels, was measured to be about 0.7 ft during 

that same period. 

The difference in the response of the two ponds to the same 

changes in recharge indicate that Duck Pond is more susceptible 

to changes in stress conditions than ponds farther from the top 

of the mound, closer to ground-water discharge areas. Two 

simulations then were made to determine the response of water 

levels in Duck Pond to in-season and off-season differences in 

recharge and proposed pumping. 

An initial simulation of off-season and in-season changes 

in recharge and pumping was made to determine the current 

range in pond level at Duck Pond for current conditions. Based 

on a simulated off-season recharge of 18 in. from October 

to April and an in-season recharge of 6 in. from May to 

September, the model-calculated annual fluctuation in pond 

levels for Duck Pond was 1.2 ft. This value is lower than the 

1.6-ft range reported by Sobczak and others (2003); possible 

reasons for the difference include temporal discretization of the 

recharge or a difference between the total recharge during 

the measurement period (1994–99) and the assumed annual 

average recharge of 24 in/yr. The 1.2-ft value for the annual 

pond fluctuation, however, will be used in this analysis as the 

baseline fluctuation for the purpose of comparing the changes 

in pond levels for different pumping rates.

A second simulation was made with off-season and inseason changes in pumping and recharge. The annual average 

pumping rates of 0.1 Mgal/d at each well were apportioned 

between off-season and in-season pumping rates of 0.04 Mgal/d 

and 0.18 Mgal/d, respectively. The wastewater discharged to 

the aquifer was specified at 85 percent of the total pumping for 

each season and added to the respective recharge rates of 18 and 

6 in/yr for off-season and in-season conditions. 

The off-season and in-season declines in the level of Duck 

Pond were 0.3 and 0.5 ft, respectively, relative to the levels 

calculated with only temporally variable recharge. Although the 

model-calculated declines in pond level for off-season and inseason pumping and recharge conditions are less than half of 

the model-calculated annual fluctuation of 1.2 ft for current 

conditions, it is important to note that these reductions are 

superimposed on the current pond-level fluctuations. As a 

result, the current model-calculated pond-level altitude ranges 

from 10.13 to 8.94 ft above NGVD 29 and from 9.84 ft to 

8.43 ft above NGVD 29 for the current and proposed pumping 

conditions (fig. 26). 

In addition to changes in the off-season and in-season 

water levels, the annual fluctuation of 1.2 ft for current 

conditions increases to 1.4 ft for proposed pumping conditions. 

The areas most affected by changes in pond levels are the plantfilled shallow-water (littoral) areas of the pond. Depending on 

the bathymetry of the pond bottom, small changes in pond 

levels may substantially affect the near-shore pond ecology 

(Portnoy and others, 2001).
Simulation of Proposed Ground-Water-Pumping Scenarios 35

Wellfleet Wellfleet

Wellfleet

Harbor

Atlantic Ocean

Duck

Pond

Dyer

Pond Great

Pond

Long

Pond

Herring River

Gull

Pond

Wellfleet

Harbor

Atlantic Ocean

Duck

Pond

Dyer

Pond

Great

Pond

Long

Pond

Herring River

Gull

Pond

-0.6

-0.5

-0.6

-0.7

-0.4

-0.5

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.1

-0.2

-0.3

-0.4 -0.4 -0.5

-0.6

9

8

7

6

5

4

3

2

9

8

7

6

5

4

3

2

-0.1

-0.2

Boy Scout Camp site

Wellfleet by

the Sea site

Boy Scout Camp site Wellfleet by

the Sea site

Coles Neck Coles Neck

70o03' 70o00'

0 5 MILES

0 5 KILOMETERS

B.

70o03' 70o00'

42o

58'

41o

55'

A.

EXPLANATION

-0.1

9

Figure 25. Location of A, model-calculated contributing area to Duck Pond and water-table contours for current (2002) 

conditions; and B, model-calculated contributing areas to Duck Pond, Coles Neck well, Boy Scout Camp site, and the 

Wellfleet By the Sea site, each pumping at 0.10 million gallons per day, and changes in model-calculated water levels from 

current (2002) conditions, Wellfleet, Massachusetts.
36 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

AVERAGE ANNUAL RECHARGE

Current (2002) pumping

Proposed pumping

SEASONALLY VARIABLE 

RECHARGE

Current (2002) pumping

Proposed pumping

MONTHS

ALTITUDE, IN FEET ABOVE NGVD 29

S O N D J F M A M J J A S

8.0

8.2

8.4

8.6

8.8

9.0

9.2

9.4

9.6

9.8

10.0

10.2

Figure 26. Model-calculated monthly pond-level altitudes in Duck Pond for current (2002) 

conditions and simulated pumping conditions of 0.10 million gallons per day at the Coles Neck well, 

the Boy Scout Camp site, and the Wellfleet By the Sea site, Wellfleet, Massachusetts.

Effects on the Movement of the 

Freshwater/Saltwater Interface

The Pamet flow lens is the source of drinking water for the 

residents of Provincetown and the residents of Truro who live 

north of the Pamet River. Provincetown is in the Pilgrim flow 

lens, but the water quality in the Pilgrim flow lens is generally 

considered unacceptable for public supply without treatment 

(Environmental Partners Group, Inc., 2002). Most Truro 

residents in the Pamet lens obtain drinking water from private 

wells. The exceptions are residents and businesses along Route 

6 and 6A (TAZ 8, fig. 12). 

In response to concerns about increased water demand 

from Provincetown, the potential for future public-supply 

needs in Truro, and increases in the peak summer demand, 

the towns of Provincetown and Truro have identified two 

potential sites to investigate for possible future water supply, 

the North Unionfield and CCC-5 sites (fig. 12). In addition, the 

Provincetown Water Department is evaluating the potential for 

increasing the current pumping at the South Hollow well field 

from an average of 0.57 to 0.8 Mgal/d. 

Simulations were conducted to evaluate the effects 

of future pumping on the position and movement of the 

freshwater/saltwater interface in the Pamet flow lens. Each of 

the proposed wells was pumped at 0.8 Mgal/d for 48 years 

(2002–50) in conjunction with the existing wells, each of which 

was pumped at its current (2002) rate. It was assumed that all 

the water pumped from these proposed sites will be used in 

Provincetown, and, therefore, will be removed from the Pamet 

flow lens and recharged to the Pilgrim flow lens through septic 

systems and a sewage-treatment facility. 

Changes in the depth to the freshwater/saltwater interface 

were determined by comparing the interface positions 

calculated for the year 2050 for the existing wells pumping at 

the current rates to the positions calculated with the addition of 

the new wells pumping for the same period. Sea level was set at 

0.63 ft above NGVD 29 (its current altitude) for the simulation 

period because the purpose of this analysis was to compare the 

effect of the different pumping scenarios on the position of the 

freshwater/saltwater interface. 
Simulation of Proposed Ground-Water-Pumping Scenarios 37

In the first simulation, the potential well site at North 

Unionfield, which is about 3,000 ft south of the South Hollow 

well field and about 3,000 ft west of the NTAFB (fig. 12), 

was pumped at 0.8 Mgal/d. After 48 years of pumping, the 

altitude of the freshwater/saltwater interface beneath the 

North Unionfield well increased by about 58 ft from a modelcalculated pre-pumping altitude of about 254 ft below 

NGVD 29. Of the 58 ft of rise, 2 ft is the response of the 

interface to current (2002) pumping at the existing wells and the 

remaining 56 ft is the result of pumping at the North Unionfield 

well. 

The effect of pumping on the freshwater/saltwater 

interface extends beyond the North Unionfield site to areas 

beneath the existing wells at South Hollow and the NTAFB 

wells 4 and 5. Beneath the South Hollow well field, the altitude 

of the interface would increase by about an additional 17 ft 

above the increase of 10 ft calculated for continuing the current 

pumping at the existing wells for the next 48 years. Beneath 

NTAFB wells 4 and 5, the altitude of the interface would 

increase by about an additional 26 ft and 21 ft, respectively, 

above the increase of 8 ft calculated for each well for continuing 

the current pumping at the existing wells for the next 48 years. 

These increases are greater than those calculated for the South 

Hollow well field. The potential effect on the NTAFB wells is 

greater because the measured interface positions at these wells 

for current conditions are 118 ft and 67 ft below the depths of 

the well screens, compared to about 150 ft at the South Hollow 

well field. 

The proximity of the North Unionfield site to the existing 

wells at South Hollow and the NTAFB also affects the 

contributing areas to these well fields. Because the North 

Unionfield site is upgradient of the existing wells and closer to 

the top of the water-table mound, pumping at the site will result 

in the capture of water that otherwise would have discharged to 

the existing wells. Thus, in order for the existing wells to satisfy 

their current pumping rates, these wells will begin to capture 

water from areas not discharging to North Unionfield. The 

contributing areas to these wells will shift from their current 

locations and bend around the contributing area to the proposed 

North Unionfield well site (fig. 27). 

An additional simulation was made to evaluate the effect 

of increasing pumping from 2002 to 2050 at South Hollow from 

0.57 Mgal/d to 0.80 Mgal/d with a constant sea level of 0.63 ft 

above NGVD 29. In this simulation, only the existing well 

fields at South Hollow and the North Truro Air Force Base were 

pumped with the South Hollow wells pumped at the increased 

rate of 0.80 Mgal/d. In response to the increase in pumping at 

the South Hollow well field, the altitude of the freshwater/ 

saltwater interface increased by about 19 ft above the altitude 

calculated for a constant pumping rate of 0.57 Mgal/d for the 

same period. This increase is similar to the increase calculated 

for the well field when pumping for a constant pumping rate of 

0.57 Mgal/d and for the North Unionfield site for a constant rate 

of 0.80 Mgal/d. The altitude of the freshwater/saltwater 

interface only increased by 2 ft beneath NTAFB well 4 and 

there was no effect calculated beneath NTAFB well 5. These 

calculated changes were minimal when compared to 26 and 21 

ft of increase calculated for these wells, respectively, in 

response to pumping at the North Unionfield site. 

The increase in pumping at South Hollow from 

0.57 Mgal/d to 0.80 Mgal/d produced a corresponding increase 

in the size of the area contributing recharge to the well 

(figs. 27A and B); however, this increase in the area contributing 

recharge to the South Hollow well field did not substantially 

affect the areas contributing recharge to the NTAFB wells 

unlike the effects from pumping at the North Unionfield site 

(fig. 27C). 

In the next simulation, the proposed well site at CCC-5 

about 8,000 ft south of the North Unionfield site (fig. 12) was 

pumped at 0.80 Mgal/d. Again, sea level was assumed to be 

0.63 ft above NGVD 29. After 48 years of pumping, the altitude 

of the interface beneath CCC-5 increased by about 14 ft from a 

model-calculated pre-pumping altitude of about 250 ft below 

NGVD 29. 

The simulated effect of pumping at the CCC-5 site does 

not extend far enough north to affect substantially the 

contributing areas (fig. 28) or the freshwater/saltwater interface 

position beneath the existing well fields. The altitude of the 

freshwater/saltwater interface increased by only 1 ft at the 

South Hollow well field, 2 ft at NTAFB well 4, and 3 ft at 

NTAFB well 5. Simulated pumping from CCC-5, however, 

did decrease discharge at the Little Pamet River from 0.62 to 

0.42 ft3/s, or a 32-percent reduction in flow from current 

conditions. The reason for this reduction in streamflow is that 

pumping from the CCC-5 site captures water that otherwise 

would have discharged to the stream (fig. 28); this scenario is 

similar to the simulated pumping at the Roach site in the town 

of Eastham and its effect on Hatches Creek (fig. 23B). 

On the basis of the results of these simulations, it was 

determined that the simulated response of the freshwater/ 

saltwater interface to changes in pumping differed throughout 

the Pamet flow lens. The three pumping sites—South Hollow 

well field, the North Unionfield site, and the CCC-5 site—have 

similar pre-pumping altitudes for the freshwater/saltwater 

interface and yet the responses of the interface to pumping 

differed greatly among the sites. A possible explanation is 

that the altitude of the pre-pumping freshwater/saltwater 

interface may be insensitive to the aquifer properties used 

in the simulations to represent variability in the subsurface 

hydrogeology. The calculated response of the freshwater/ 

saltwater interface to changes in pumping at these locations, 

however, may be affected greatly by simulated aquifer 

properties. For example, the pre-pumping altitude of the 

freshwater/saltwater interface beneath the South Hollow

well field was calculated to be 230 ft below NGVD 29. 
38 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

EXPLANATION

2

A. B.

C.

6

5

4

3

2

6

5

4

3

2

42o02'30"

42o

00'

70o

05' 70o

02'

6

5

4

3

2

6

5

4

3

2

70o05' 70o

02'

6

5

4

3

2

6

5

4

3

42o02'30"

42o

00'

70o

05' 70o

02'

3,000 FEET

1,000 METERS

0

0

Figure 27. Model-calculated water-table contours and contributing areas to A, South Hollow well field and the North 

Truro Air Force Base wells 4 and 5 for current (2002) pumping rates; B, South Hollow well field pumping at 0.80 million 

gallons per day and North Truro Air Force Base wells 4 and 5 pumping at current (2002) rates; and C, South Hollow well 

field and the North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates and North Unionfield site 

pumping at 0.80 million gallons per day, North Truro, Massachusetts.
Simulation of Proposed Ground-Water-Pumping Scenarios 39

6

5

4

3

2

6

5

4

3

2

42o02'30"

42o

00'

70o

05' 70o

02' 70o

05' 70o02'

6

5

4

3

6

5

2

EXPLANATION

5,000 FEET

3,000 METERS

0

0

2

A. B.

Figure 28. Model-calculated water-table contours and contributing areas to A, Little Pamet River, South Hollow well field, and 

North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates; and B, Little Pamet River, South Hollow well field, and 

North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates and CCC-5 site pumping at 0.8 million gallons per day, 

Truro, Massachusetts.

The altitude of the interface increased by 43 ft for the 

47-year period from 1955 to 2002. This change was calculated 

by simulating the actual annual average pumping rates at 

the well field, which on average was about 0.44 Mgal/d. By 

comparison, the model-calculated altitude of the freshwater/ 

saltwater interface at the North Unionfield site was 254 ft below 

NGVD 29 for pre-pumping conditions and increased by 58 ft 

with a constant pumping rate of 0.80 Mgal/d for the 48-year 

period from 2002 to 2050. 

At the CCC-5 site, the altitude of the freshwater/

saltwater interface for pre-pumping conditions was 250 ft 

below NGVD 29 and increased by only 14 ft for the same 

pumping rate and time as that of the North Unionfield site. The 

reason for the different responses of the interface altitude to 

pumping at the two sites can be attributed to the differences in 

lithology, and, therefore, differences in simulated hydraulic 

conductivity within the Pamet flow lens. 
40 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Beneath the South Hollow well field, the area in which 

the freshwater/saltwater interface shows the greatest response 

to increased pumping, the simulated transmissivity is about 

35,000 ft2/d. The freshwater/saltwater interface for prepumping conditions is 230 ft below NGVD 29 in a part of 

the aquifer with an assumed horizontal hydraulic conductivity 

of 70 ft/d and a ratio of horizontal to vertical hydraulic 

conductivity of 30:1. Beneath the North Unionfield site, the 

model-simulated transmissivity is about 22,000 ft2/d and, for 

current conditions, the freshwater/saltwater interface is about 

254 ft below NGVD 29. At this altitude, the model-simulated 

horizontal hydraulic conductivity is 10 ft/d with a ratio of 

horizontal to vertical hydraulic conductivity of 100:1. Beneath 

the CCC-5 site, the model-simulated transmissivity is about 

20,000 ft2/d and for current, pre-pumping conditions the 

freshwater/saltwater interface is about 250 ft below NGVD 29. 

At this depth, the model-simulated horizontal hydraulic 

conductivity is 10 ft/d with a ratio of horizontal to vertical 

hydraulic conductivity of 100:1. 

An aquifer test was conducted in well TSW-200 (fig. 1) 

near the CCC-5 site. The results from this aquifer test indicate 

that the transmissivity in the upper 100 ft of aquifer is about 

22,000 ft2/d with a ratio of horizontal to vertical hydraulic 

conductivity of 5:1 (Guswa and Londquist, 1976). At depths 

greater than 100 ft (about 95 ft below NGVD 29), the aquifer 

sediments consist of fine sand and silt. Therefore, the aquifer 

was assumed to have a much lower transmissivity and a much 

higher ratio of horizontal to vertical hydraulic conductivity at 

depth (Guswa and Londquist, 1976). 

The measured depth to the center of the transition zone 

between freshwater and saltwater beneath TSW-200 is about 

230 ft below NGVD 29. This measured freshwater/saltwater 

interface position is consistent with the model-calculated 

position of 247 ft below NGVD 29 beneath this well. 

Unfortunately, a measured response of the freshwater/saltwater 

interface to pumping cannot be obtained from the aquifer test to 

compare to the model-calculated results because the slow 

response in possible movement of the freshwater/saltwater 

interface greatly exceeds the length of time of the aquifer test. 

Model-simulation results indicate that the initial pre-pumping 

position of the interface may be insensitive to the simulated 

changes in subsurface lithology, as shown by the similar depths 

to the freshwater/saltwater interface beneath the South Hollow 

well field, the North Unionfield site, and the CCC-5 site despite 

large differences in aquifer transmissivity among the sites. The 

response of the freshwater/saltwater interface to changes in 

pumping for a given period, however, does appear to be 

sensitive to the simulated hydraulic properties. 

The greatest response for the 48-year period (2002–50) 

occurs under the South Hollow well field, where it is assumed 

that the aquifer is more permeable at depth and the ratio of 

horizontal to vertical hydraulic conductivity is the lowest. The 

smallest response to increased pumping was calculated for the 

CCC-5 site, where the overall transmissivity is about 40 percent 

less than that simulated at the South Hollow well field. In 

addition, the aquifer beneath the CCC-5 site is much less 

permeable and the ratio of horizontal to vertical hydraulic 

conductivity is much higher than beneath the South Hollow 

well field. 

Differences in aquifer transmissivity and in the hydraulic 

conductivity in the vicinity of the freshwater/saltwater 

interface, however, do not explain the difference in the 

responses of the freshwater/saltwater interface between the 

CCC-5 and North Unionfield sites. At these two sites, the 

simulated aquifer transmissivity values are similar (22,000 

and 20,000 ft2/d, respectively) and the horizontal hydraulic 

conductivity of the transition zones are the same, yet the modelcalculated altitude of the freshwater/saltwater interface beneath 

the North Unionfield site increased by 58 ft, compared to only 

14 ft beneath the CCC-5 site. The proximity of the North 

Unionfield site to the South Hollow and NTAFB wells may 

cause an increased effect on the altitude of the freshwater/ 

saltwater interface beneath the well site. 

Another possible explanation for the difference in 

the responses of the freshwater/saltwater interface may 

be the distribution of less permeable material beneath the 

pumping wells between the bottom of the well screen and 

the freshwater/saltwater interface rather than the overall 

transmissivity as determined by an aquifer test. There is a zone 

beneath the North Unionfield site about 90 ft thick with a 

simulated hydraulic conductivity value of 30 ft/d between the 

bottom of the proposed pumping well and the freshwater/ 

saltwater interface. At the CCC-5 site, this zone of low 

hydraulic conductivity is assumed to be 150 ft thick. This zone 

could affect the response of the freshwater/saltwater interface to 

pumping at the CCC-5 site. 

During a typical aquifer test, changes in water levels 

are measured in response to pumping over a specified period. 

The slow response of the freshwater/saltwater interface to 

changes in pumping, however, makes it impractical to measure 

the change in the freshwater/saltwater interface position in 

response to changes in pumping in the field. Therefore, 

analytical equations and numerical flow models that 

incorporate the available information on the subsurface 

hydrogeology must be used to predict the future effects of 

pumping on the movement of the freshwater/saltwater 

interface. 

In this investigation, the assumptions about the 

distribution of aquifer properties throughout the flow lens are 

based on limited field data augmented by the relation between 

the regional depositional model of the glacial material and the 

model-simulated hydraulic-conductivity values. These results 

underscore the importance of developing a better understanding 

of the hydraulic properties of the subsurface materials through 

(1) well-designed aquifer tests, (2) detailed characterization of 

the lithology through the vertical extent of the aquifer down to 

the freshwater/saltwater interface, and (3) measurements of the 

pre-pumping depth to the freshwater/saltwater interface, prior 

to developing large-capacity pumping wells at the proposed 

well sites.
Simulated Effects of Local Sea-Level Change Through Removal of a Tide-Control Structure 41

Simulated Effects of Local Sea-Level 

Change Through Removal of a 

Tide-Control Structure

Tide-control structures were built along the mouths of 

river systems throughout Lower Cape Cod, Massachusetts, over 

the past 150 years to control mosquitoes, reclaim estuary land, 

and provide flood control. Roman and others (1987) determined 

that the tide-control structures in these river systems have 

resulted in the unintended effects of fish kills and increased 

mosquito-control problems. As a result, the National Park 

Service (NPS) is assessing the potential hydrological and 

ecological effects of removing the tide-control structures and 

restoring the river systems to their natural state, and will 

monitor the results of restoration projects to determine whether 

expectations are being met.

The removal of tide-control structures may affect 

hydraulic conditions in the upland areas adjacent to the areas of 

increased tidal flux. Ground-water altitudes, the depth to the 

interface between freshwater and saltwater, and ground-water 

discharge to freshwater streams may change. The effect on 

domestic water-supply wells in these areas will depend on 

well depths and the magnitude of the hydraulic changes. 

A previous USGS investigation near the Herring 

River tide-control structure in Wellfleet, Massachusetts 

(fig. 1) (Fitterman and Dennehy, 1991), used surficial and 

borehole geophysical techniques to characterize the depth to the 

freshwater/saltwater interface and to assess the potential effect 

of the removal of the tide-control structure on the interface in 

the vicinity of an area of predicted increased tidal flux. Based 

on a predicted rise in water level of about 1.5 ft (Roman and 

others, 1987) and the measured position of the freshwater/ 

saltwater interface (about 60 ft below NGVD 29) in the vicinity 

of Chequesset Neck (fig. 1), Fitterman and Dennehy (1991) 

concluded that the removal of the tide-control structure would 

have little effect on the total thickness of the freshwater lens. 

The conclusion reached by Fitterman and Dennehy (1991), 

based on the present-day position of the freshwater/saltwater 

interface, was not supported by numerical-model simulations of 

the effects of removing the tide-control structure on the position 

of the underlying freshwater/saltwater interface. The numerical 

model developed in this study was used to assess the response 

of the flow system to moving the location of the tide-control 

structure from the Herring River to the upper reach of Mill 

Creek (fig. 29). 

On the basis of hydrodynamic and salinity modeling, 

Spaulding and Grilli (2001) predicted that the removal of the 

two tidal gates and the complete opening of the sluice way on 

the tide-control structure on the Herring River [referred to as 

"case 5" in Spaulding and Grilli (2001)] will result in a 

landward flooding of seawater to an altitude at mean tide 

of about 2.5 ft above NGVD 29 as far inland as High Toss 

Road (fig. 29). Under current conditions (2002), the altitude 

of the Herring River is about 0.7 ft above NGVD 29 on the 

landward side of the tide-control structure (Spaulding and 

Grilli, 2001). If the structure is removed, the increases in stream 

stage and salinity may result in additional flooding of saline 

water into the lowland areas of Chequesset Neck in the vicinity 

of Mill Creek; these areas were salt marshes prior to the 

installation of the tidal restriction in 1907 (fig. 29B, see 

saltwater head-dependent discharge boundaries). 

The mean sea level measured in September 2000 at the 

seaward side of the Herring River tide-control structure was 

about 1.5 ft above NGVD 29 (Spaulding and Grilli, 2001). The 

model was modified for this analysis by increasing the current 

sea-level position from 0.63 ft to 1.5 ft above NGVD 29 in the 

saltwater head-dependent discharge boundaries near the mouth 

of the Herring River (fig. 29B) in order to be consistent with the 

tidal position measured by Spaulding and Grilli (2001) at the 

mouth of the river. In addition, the pumping well used to irrigate 

the Chequesset Country Club golf course (fig. 29A, Site 3) was 

added to the model with an average annual pumping rate of 

0.015 Mgal/d from a screened interval of 20 to 40 ft below 

NGVD 29. 

A simulation was made with the golf-course well pumping 

at 0.015 Mgal/d and the new tidal altitude in the vicinity of 

the mouth of the Herring River set at 1.5 ft above NGVD 29 

to establish the current position of the freshwater/saltwater 

interface. Similar modifications to the regional model, such as 

the simulated coastal boundary condition, could be made at 

other tidally restricted inlets and coves throughout the Lower 

Cape Cod study area if the tidal-altitude information was 

available; however, measuring these tidal positions throughout 

the study area was beyond the scope of the regional 

investigation. 

Four scenarios were simulated to assess the potential 

effects of removing the tide-control structure on the position of 

the freshwater/saltwater interface in the vicinity of the Herring 

River. These simulations were made for a period of about 300 

years from present to allow enough time for the flow system to 

reach equilibrium with respect to changes in the position of the 

freshwater/saltwater interface. 

Five sites were selected at which to evaluate changes in the 

position of the interface in relation to the simulated current 

position of the interface (fig. 29A). Sites 1 and 2 are the 

locations of monitoring wells installed during the investigation 

conducted by Fitterman and Dennehy (1991). Two of the 

remaining three sites (Sites 3 and 4) are the locations of 

monitoring wells installed by the USGS in September 2003 to 

assist the NPS in monitoring possible changes in the interface 

from the removal of the tide-control structure. 
42 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Base from U.S. Geological Survey topographic quadrangles, Wellfleet, and Orleans, Massachusetts, Universal Transverse 

Mercator grid, Polyconic projection, zone 19 NAD 27,

1:25,000

Site 1

Site 2

Site 4

Site 3

Site 5

70o04' 30" 70o04'

41o

56'

41o

56'

Golf

Course

A. EXPLANATION

B.

Proposed tidal

 control structure

 Existing tidal

 control structure

High Toss Road

High Toss Road

HEAD-DEPENDENT

DISCHARGE BOUNDARY

Freshwater

Saltwater concentration, in

pounds per cubic foot 

2.18

1.63

1.09

0.36

0.18

2,000 FEET

600 METERS

0

0

Figure 29. Model-simulated boundary conditions with A, the existing Herring River tide-control structure and conditions in 

the Chequesset Neck area; and B, proposed tide-control structure and the variable saltwater concentrations used in 

simulations 1 and 4, Wellfleet, Massachusetts.
Simulated Effects of Local Sea-Level Change Through Removal of a Tide-Control Structure 43

In the first simulation, the surface-water altitude was 

specified uniformly at 2.5 ft above NGVD 29 upstream in the 

Herring River from the tide-control structure to High Toss Road 

and from the mouth of Mill Creek to the proposed site of the 

new tide-control structure (fig. 29B). The increased stage of 

2.5 ft above NGVD 29 is the stage calculated by Spaulding and 

Grilli (2001) for the removal of the two tidal gates and the 

complete opening of the sluice way on the tide-control 

structure. In addition to increasing the stage in the river and 

stream channels, new head-dependent boundary conditions 

with a water level of 2.5 ft above NGVD 29 were specified in 

the lowland areas that were salt marshes prior the installation of 

the tide-control structure in 1907 and now are occupied in part 

by the Chequesset Country Club golf course (fig. 29).

In the areas where increased tidal exchange was expected 

to increase the stream stage in Mill Creek and the Herring River 

and to flood the adjacent lowland areas, the specified saltwater 

concentrations at these head-dependent discharge boundaries 

were set equal to the concentrations calculated by Spaulding 

and Grilli (2001) (fig. 29B). The salt concentrations ranged 

from the high concentration of saltwater (2.18 lb/ft3) at the tidecontrol structure to the low concentration of freshwater (0.0 

lb/ft3) near High Toss Road.

As a result of these simulated changes in stream stage and 

salt concentration, the model-calculated altitude of the 

freshwater/saltwater interface decreased between 8 and 36 ft 

(table 2) at the five monitoring sites relative to the simulated 

current conditions, despite the fact that salt concentrations 

shown in figure 29 were specified for areas that currently are 

freshwater. 

In the second simulation, the stream stage of 2.5 ft above 

NGVD 29 was used again for the stage in Mill Creek, Herring 

River, and the adjacent flooded lowland areas. For the salt 

concentration, however, seawater was specified for all model 

cells assigned a head-dependent discharge boundary condition 

(see appendix), rather than the gradient shown in figure 29B and 

used in the first simulation. 

Results from the second simulation show that the change 

in the specified salt concentration from that shown in figure 29B

has a large effect on the position of the freshwater/saltwater 

interface. The altitude of the interface rose between 19 and 80 

ft beneath the five monitoring sites (table 2) relative to the 

simulated current altitude. 

In the third simulation, the salt concentration was specified 

to be the same as that of the second simulation. In this 

simulation, however, the stream stage in Mill Creek, the 

Herring River, and the adjacent flooded lowland areas was 

uniformly specified at 1.5 ft, the stage that would occur if the 

entire tide-control structure was removed, rather than only the 

tidal gates. In this simulation the altitude of the freshwater/ 

saltwater interface increased between 3 and 8 ft beneath the five 

monitoring sites (table 2) relative to the current altitude. This 

effect was less than that calculated in the previous simulation 

with the same specified salt concentrations but a higher 

specified stage. 

In the fourth simulation, a uniform stream stage of 1.5 ft 

above NGVD 29 was specified for Mill Creek, the Herring 

River, and the adjacent flooded lowland areas. The salt 

concentration specified in this simulation, however, was the 

gradient specified in the first simulation (fig. 29B). In this 

simulation, the altitude of the freshwater/saltwater interface 

increased between 5 and 10 ft beneath the five monitoring sites 

(table 2) relative to the simulated current altitude.

The results of the four model simulations show that the 

specified salt concentrations and the increased stream stage 

caused by the removal of the tidal gates and/or the tide-control 

structure can affect the simulated position of the 

freshwater/saltwater interface. The simulated thickness of the 

freshwater lens in the vicinity of the Herring River depended 

upon the combination of specified stream stage and salt 

concentration and changed considerably from the thickness 

simulated under current (2002) conditions.

Table 2. Model-calculated changes in the altitude of the freshwater/saltwater interface in the vicinity of the Herring River tide-control 

structure, Wellfleet, Massachusetts, in response to changes in simulated salt concentrations and stream stage

[All altitudes in feet above NGVD 29. Well site with model-cell location: Numbers in parentheses next to well-site number are model cell (row, column).]

Simulation Salinity Surface-water 

altitude, in feet

Well site with model-cell location

1 (174, 57) 2 (177, 56) 3 (181, 49) 4 (178, 52) 5 (182, 53)

Current Fresh 0.7–1.5 -111 -119 -62 -80 -85

1 Variable 2.5 -119 -128 -81 -116 -100

2 Salt 2.5 -31 -82 -43 -46 -29

3 Salt 1.5 -103 -111 -55 -77 -80

4 Variable 1.5 -103 -110 -52 -72 -80
44 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

The results from the first two simulations indicate that, 

with the higher value of the stage height, the position of the 

freshwater/saltwater interface is sensitive to the specified salt 

concentration. When in addition, it was assumed that the salt 

concentration gradually decreased landward of the tide-control 

structure (Spaulding and Grilli, 2001) (fig. 29B), the altitude 

of the freshwater/saltwater interface decreased. Thus, the 

freshwater lens became thicker in the vicinity of the Herring 

River. When the salt concentration was specified to equal the 

concentration of seawater, the altitude of the freshwater/ 

saltwater increased. Thus, the freshwater lens became thinner 

in the vicinity of the Herring River. 

In the third and fourth simulations, the stream stage was 

set at 1.5 ft above NGVD 29, the current altitude of seawater 

measured on the seaward side of the tide-control structure. In 

these simulations, it was assumed that the entire tide-control 

structure was removed and the stream stage as far inland as 

High Toss Road is the same as the current altitude of seawater. 

The salt concentrations were varied from a uniform 

concentration similar to seawater from the tidal restriction to 

High Toss Road (simulation 3) to the concentration gradient 

predicted by Spaulding and Grilli (2001) (simulation 4). In 

these simulations, the altitude of the freshwater/saltwater 

interface increased by less than 10 ft (table 2) relative to 

the current altitude. The difference in the salt concentration 

between simulations 3 and 4 did not appear to have a substantial 

effect on the position of the freshwater/saltwater interface. 

The specified salt concentrations appeared to have the 

greatest effect on the simulated position of the freshwater/ 

saltwater interface when the stream stage was set at 2.5 ft above 

NGVD 29, the predicted mean tide altitude if only the tidal 

gates were removed from the tide-control structure (Spaulding 

and Grilli, 2001). When the stream stage was set at 1.5 ft above 

NGVD 29, however, the position of the freshwater/saltwater 

interface did not appear to be affected greatly by the change in 

specified salt concentrations.

The results of these simulations indicate that setting the 

stream stage in the Herring River, Mill Creek, and the adjacent 

lowland areas at an altitude of 2.5 ft above NGVD 29 resulted 

in a decrease in ground-water discharge and a subsequent rise 

in the water table in the vicinity of the Herring River. When 

the specified salt concentrations were spatially variable and 

low, such as those predicted by Spaulding and Grilli (2001) 

(fig. 29B), and the stream stage was set at 2.5 ft above 

NGVD 29, the altitude of the freshwater/saltwater interface 

decreased. When the simulated seawater concentration was 

high and the stream stage was set at 2.5 ft above NGVD 29, 

however, the altitude of the interface rose and resulted in the 

thinning of the freshwater lens with time. It should be noted that 

this simulation represents a worst-case scenario with respect to 

the movement of the freshwater/saltwater interface and that the 

hydrodynamic modeling simulations of Spaulding and Grilli 

(2001) do not predict a salt concentration of seawater as far 

inland as High Toss Road. 

When the stream stage was set at the present sea-level 

altitude of 1.5 ft above NGVD 29, the altitude of the 

freshwater/saltwater interface increased slightly from the 

current altitude regardless of the specified salt concentration. 

The reason for the small response in the interface to the 

increased stream stage of 1.5 ft above NGVD 29 may be that 

this stream stage is similar to the present water levels on both 

the landward and seaward sides of the tide-control structure 

and, thus, the changes in salt concentrations resulted in only 

small changes in the interface position. 

Summary

Federal, State, and local officials responsible for managing 

and protecting water resources of the Lower Cape Cod aquifer 

system are concerned that a shift to large-capacity, centralized 

municipal supplies from private small-capacity wells may 

create the potential for unacceptable declines in water table and 

pond altitudes, decreases in ground-water discharge to streams 

and coastal areas, and saltwater intrusion. In response to these 

concerns, the U.S. Geological Survey (USGS) in cooperation 

with the National Park Service, Massachusetts Executive Office 

of Environmental Affairs, Cape Cod Commission, and the 

Towns of Eastham, Wellfleet, Truro, and Provincetown began 

an investigation in 2000 to improve the understanding of the 

hydrogeology of the Lower Cape Cod aquifer system and to 

assess the effects of changing ground-water pumping and 

recharge conditions on ground-water flow in Lower Cape Cod.

A regional numerical flow model was developed as part of 

this investigation to assist in the analysis of freshwater and 

saltwater flow for changing pumping, recharge, and sea-level 

conditions. The model was used to determine water budgets, 

flow directions, and the position and movement of the 

freshwater/saltwater interface throughout the study area for 

current conditions. The model also was used to assess potential 

effects of increased pumping for proposed pumping well 

locations on ponds, streams, coastal areas, and the position of 

the freshwater/saltwater interface to demonstrate how the 

model can serve as a tool that can be used by the State and local 

managers to assess possible effects of proposed watermanagement strategies on Lower Cape Cod.
References Cited 45

The primary results of the investigation are listed below:

• Of the approximately 68 Mgal/d of freshwater that 

recharges the aquifer system, about 68 percent of this 

water moves through the ground-water flow system 

and discharges directly to the coast, 31 percent reaches 

the coast as streamflow, and 1 percent discharges to 

public-supply wells. The distribution of streamflow 

varies greatly between flow lenses. In the Chequesset 

flow lens, for example, streamflow accounts for over 

half of the total discharge to the coast, whereas in the 

Pilgrim flow lens, streamflow accounts for less than 10 

percent of the total discharge to the coast.

• The depth to the freshwater/saltwater interface varies 

throughout the study area and is largely controlled by 

the areal extent of the recharge areas of the flow lenses. 

In general, the larger the area receiving recharge, the 

greater the depth to the freshwater/saltwater interface; 

however, changes in subsurface geology or the 

presence of streams can affect this relation. In areas 

such as the western portion of the Chequesset flow lens, 

the depth to the freshwater/saltwater interface is 

shallow compared to the eastern portion of the lens 

where no streams are present. In the Nauset flow lens, 

which is underlain by thick deposits of silt and clay, the 

water-table altitude is higher than in the other flow 

lenses and the depth to the freshwater/saltwater 

interface is greater.

• Sea-level rise may increase water levels and 

streamflows throughout the Lower Cape Cod aquifer 

system, and yet decrease the depth to the 

freshwater/saltwater interface. The amount of increase 

in water levels in response to sea-level rise is affected 

by non-tidal streams because increases in streamflow 

limit the water-level increase. In areas where the waterlevel rise is limited by increased streamflow, the depth 

to the freshwater/saltwater interface decreases.

• Pumping from large-capacity municipal-supply wells 

increases the potential for effects on surface-water 

bodies, and, therefore, requires careful planning of 

future pumping and wastewater-disposal locations and 

rates. Pumping wells that are upgradient of surfacewater bodies have the potential to capture water that 

would otherwise discharge to these surface-water 

bodies, thereby reducing streamflow and pond levels. 

Kettle-hole ponds, such as Duck Pond in Wellfleet, that 

are located near the top of a water-table mound, appear 

to be more susceptible to changing pumping and 

recharge conditions than the kettle-hole ponds closer to 

the coast or near streams. 

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application at Truro wellfield, Cape Cod, Massachusetts: 

Ground Water, v. 25, no. 2, p. 82–92. 

Reilly, T.E., and Goodman, A.S., 1985, Analysis of saltwater 

upconing beneath a pumping well: Journal of Hydrology, 

v. 89, p. 169–202. 

Reilly, T.E., and Pollock, D.W., 1993, Factors affecting areas 

contributing recharge in shallow aquifers: U.S. Geological 

Survey Water-Supply Paper 2412, 21 p. 

Roman, C.T., Garvine, R.E., Marteinsdottir, G., Nikula, B., 

Nuttle, W.K., and Orson, R.A., 1987, An evaluation of 

alternatives for estuarine restoration management—The 

Herring River ecosystem (Cape Cod National Seashore): 

New Brunswick, NJ, Center for Coastal Environmental 

Studies, Rutgers–The State University of New Jersey, 304 p. 

Smith, N.D., and Ashley, G.M., 1985, Proglacial lacustrine 

environment, in Ashley, G.M., Shaw, J., and Smith, N.D., 

eds., Glacial sedimentary environments: Tulsa, OK, Society 

of Paleotologists and Mineralogists, Short Course No. 16, 

246 p. 

Sobczak, R.V., and Cambareri, T.C., 1998, Water resources of 

outer Cape Cod—Final report of the Lower Cape Water 

Management Task Force: Barnstable, MA, Cape Cod 

Commission, 94 p. 

Sobczak, R.V., Cambareri, T.C., and Portnoy, J.W., 2003, 

Physical hydrology of selected vernal pools and kettle hole 

ponds in the Cape Cod National Seashore, Massachusetts: 

Barnstable, MA, Cape Cod Commission, 174 p. 

Spaulding, M.L., and A. Grilli, 2001, Hydrodynamic and 

salinity modeling for estuarine habitat restoration at Herring 

River, Wellfleet, Massachusetts: Prepared for the National 

Park Service, U.S. Department of Interior, Wellfleet, MA, 

Cape Cod National Seashore, 93 p. 

Strahler, A.N., 1972, The environmental impact of groundwater use on Cape Cod, Impact Study III: Orleans, MA, 

Association for the Preservation of Cape Cod, 68 p. 

Theiler, E.R., and Hammar-Klose, E.S., 2000, National 

assessment of coastal vulnerability to sea-level rise; 

Preliminary results for the U.S. Atlantic coast: U.S. 

Geological Survey Open-File Report 99-593, 1 p. 

Walter, D.A., Masterson, J.P., and LeBlanc, D.R., 2002, 

Simulated pond-aquifer interactions under natural and 

stressed conditions near Snake Pond, Cape Cod, 

Massachusetts: U.S. Geological Survey Water-Resources 

Investigations Report 99-4174, 35 p. 

Uchupi, Elazar, Giese, G.S., and Aubrey, D.G., 1996, The late 

Quaternary construction of Cape Cod, Massachusetts—A 

reconsideration of the W.M. Davis Model: Geological 

Society of America Special Paper 309, 69 p. 

Woodworth, J.B., and Wigglesworth, Edward, 1934, 

Geography and geology of the region including Cape Cod, 

the Elizabeth Islands, Nantucket, Martha’s Vineyard, No 

Mans Land, and Block Island: Harvard College Museum of 

Comparative Zoology Memoir 52, 332 p. 

Zeigler, J.M., Tuttle, S.D., Tasha, H.J., and Giese, G.S., 1965, 

The age and development of the Provincelands Hook, outer 

Cape Cod, Massachusetts: Limnology and Oceanography, 

Redfield Volume, p. R298–R311.

APPENDIX: DEVELOPMENT OF 

GROUND-WATER MODEL

Contents 51

Contents

Development of Ground-Water Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

Model Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

Boundary Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

Model Stresses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

Recharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57

Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59

Model Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72

Figures

1-1–1-5. Maps showing:

1-1. Model extent and distribution of simulated boundary conditions of ground-water-flow 

model of Lower Cape Cod, Massachusetts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

1-2. Locations of long-term observation wells and Cape Cod National Seashore coastal 

ecosystem wells, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

1-3. Locations of observation wells measured in November 1975, Lower Cape Cod. . . . . . . . . . . . . . . . . .61

1-4. Locations of pond and stream-gaging sites, zone-of-transition monitoring wells, and line 

of marine resistivity measurements, Lower Cape Cod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

1-5. Locations of observation wells, water-table altitude, and configuration, May 2002, Lower 

Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

1-6, 1-7. Graphs showing:

1-6. Water-level altitudes at long-term observation wells EGW-36, PZW-78, TSW-216, TSW-89, 

WNW-17, and WNW-30, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

1-7. Profiles of electromagnetic (EM) geophysical logs at A, Knowles Crossing; and B, North 

Truro Air Force Base wells 4 and 5 measured in September 2000 and model-calculated 

changes in salt concentration with depth, Truro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

1-8. Cross sections showing A, the marine streaming resistivity profile and B, the model-calculated 

boundary between freshwater and saltwater, Eastham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

Tables

1-1. Vertical layers and horizontal and vertical hydraulic conductivity for the flow model of the Lower 

Cape Cod aquifer system, Cape Cod, Massachusetts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

1-2. Annual average pumping rates for Provincetown Water Department public-supply wells from 

1907 to 2002, Truro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58

1-3. Measured water levels for selected observation wells and kettle-hole ponds in the modeled 

area in November 1975 and June 2001, and the median water-level altitudes of the long-term 

observation wells for the measured period of record and differences between these and 

model-calculated water-level altitudes for simulated current (2002) pumping and recharge 

conditions, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66

1-4. Measured discharge for selected streams in the modeled area in September 2000 and May 

2002, and model-calculated streamflows for simulated current (2002) pumping and recharge 

conditions, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68

1-5. Measured depth to the transition between freshwater and saltwater at selected zone-oftransition wells from LeBlanc and others (1986) and the model-calculated freshwater/saltwater 

interface for current (2002) pumping and recharge conditions, Lower Cape Cod . . . . . . . . . . . . . . . . . . . . .68

Development of Ground-Water Model 53

Development of Ground-Water 

Model

A numerical model of ground-water flow was developed 

to simulate freshwater and saltwater flow in the four flow lenses 

of Lower Cape Cod, Massachusetts. This model is based on 

the USGS computer program SEAWAT (Guo and Langevin, 

2002), which simulates variable-density, transient groundwater flow in three dimensions. A USGS particle-tracking 

program (MODPATH) developed by Pollock (1994) was used 

to calculate the initial locations of water particles that discharge 

to public-supply wells, ponds, streams, and coastal areas under 

changing pumping and recharge scenarios.

Model Grid

The finite-difference grid for the numerical model consists 

of uniformly spaced model cells that are 400 ft on a side. The 

grid consists of 320 rows and 110 columns and extends across 

all four flow lenses. The model has 23 layers that extend from 

the water table to a uniform depth of 500 ft below NGVD 29 

with the thickness of the vertical layers ranging from 15 to 25 ft 

(table 1-1). 

Although the unconsolidated sand-and-gravel and silt-andclay sediments that constitute the Lower Cape Cod aquifer 

system extend to bedrock, the depth to bedrock is in most places 

much greater than the depth to the freshwater/saltwater 

interface. Therefore, the freshwater flow system is bounded 

below by the transition zone between freshwater and underlying 

saltwater rather than bedrock, as is the case in the upper and 

middle parts of Cape Cod (Guswa and LeBlanc, 1985; LeBlanc 

and others, 1986; Masterson and Barlow, 1996). 

Boundary Conditions

The boundaries of the numerical model of fresh groundwater flow in the Lower Cape Cod aquifer system coincide with 

the physical boundaries of the flow system. The upper boundary 

of the model is the water table, which is a free surface boundary 

that receives spatially variable recharge from precipitation and 

wastewater disposal. The lower boundary of the freshwater 

flow system is the transition between freshwater and saltwater; 

this boundary is calculated by the numerical model. The 

arbitrary bottom altitude of 500 ft below NGVD 29 was 

specified as a no-flow boundary for this analysis; this altitude 

generally is above the top of the bedrock surface through most

of Lower Cape Cod (Oldale, 1969; B.D. Stone, U.S. Geological 

Survey, written commun., 1992) and yet is sufficiently deep not 

to affect the model-calculated position and movement of the 

freshwater/saltwater interface beneath the four flow lenses of 

Lower Cape Cod.

The lateral boundaries of the model are the coastal 

discharge areas that surround the study area. These discharge 

areas are represented as head-dependent flux boundaries that 

extend out into Cape Cod Bay and the Atlantic Ocean in the top 

three layers of the model (fig. 1-1). The distribution of these 

boundaries in the upper three layers is based on estimates of the 

offshore seabed bathymetry. 

Table 1-1. Vertical layers and horizontal and vertical hydraulic 

conductivity for the flow model of the Lower Cape Cod aquifer 

system, Cape Cod, Massachusetts.

[Altitude of layer bottom: Altitude relative to NGVD 29. K, hydraulic 

conductivity; ft, foot; ft/d, foot per day]

Model 

layer

Altitude of

layer bottom

(ft)

Horizontal K

(ft/d)

Range in

vertical K

(ft/d)

1 -5 350–150 117–30

2 -20 350–30 117–0.3

3 -40 250–30 50–0.3

4 -60 250–30 50–0.3

5 -80 200–10 40–0.01

6 -100 125–10 12.5–0.01

7 -120 125–10 12.5–0.01

8 -140 125–10 12.5–0.01

9 -160 125–10 12.5–0.01

10 -180 70–10 2.33–0.01

11 -200 70–10 2.33–0.01

12 -225 70–10 2.33–0.01

13 -250 70–10 2.33–0.01

14 -275 70–10 2.33–0.01

15 -300 70–10 2.33–0.01

16 -325 70–10 2.33–0.01

17 -350 30–10 0.3–0.01

18 -375 10–10 0.01–0.01

19 -400 10–10 0.01–0.01

20 -425 10–10 0.01–0.01

21 -450 10–10 0.01–0.01

22 -475 10–10 0.01–0.01

23 -500 10–10 0.01–0.01
54 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Truro

Provincetown

Wellfleet

Eastham

Chequesset 

Neck

Pamet River

Herring Ri ver

Cape Cod Bay Atlantic Ocean

1

1

320

110

Columns

Rows

70o15' 70o

00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

EXPLANATION

Salt

Pond

Nauset

Marsh

Pilgrim

Lake

CAPE COD NATIONAL SEASHORE

PONDS, MARSHES, AND 

WETLANDS

RIVERS

SIMULATED MODEL BOUNDARIES

Constant head–Pilgrim Lake

Head-dependent flux

Streams, layer 1

Coastal discharge area, layer 1

Coastal discharge area, layer 2

Coastal discharge area, layer 3

Figure 1-1. Model extent and distribution of simulated boundary conditions of ground-water-flow model of Lower Cape Cod, 

Massachusetts.
Development of Ground-Water Model 55

Because these boundaries represent saltwater, an initial 

salt concentration representative of seawater (2.18 lbs/ft3) was 

specified for these model cells. The freshwater heads specified 

as part of these boundary conditions were calculated as 

equivalent freshwater heads, thereby creating hydrostatic 

conditions with depth for this coastal boundary. These 

equivalent freshwater heads are based upon an assumed altitude 

of sea level. Analyses were conducted that adjusted sea level by 

0.104 in/yr (2.65 mm/yr) from an assumed altitude of 0.0 ft in 

1929 (NGVD 29) to an altitude of 1.05 ft above NGVD 29 in 

2050. The hydraulic conductance of the seabed deposits was 

calculated for each model cell containing a coastal boundary as 

described in Harbaugh and McDonald (1996) as 

, (1-1)

where

The vertical hydraulic conductivity (K) of the seabed 

deposits in most of the study area was assumed to be 1 ft/d, 

which is consistent with model simulations of similar coastal 

discharge areas in other areas on Cape Cod (Masterson and 

others, 1998). In the area occupied by Salt Pond and Nauset 

Marsh, it was assumed that there were thick deposits of lowpermeability material (J.A. Colman, U.S. Geological Survey, 

oral commun., 2002) and the vertical hydraulic conductivity 

was set to 0.1 ft/d. The thickness of the seabed deposits was 

assumed to be half the thickness of the model cell containing the 

boundary. 

In addition to head-dependent boundaries specified 

along coastal discharge areas, SEAWAT requires that headdependent flux boundaries be specified along the lateral extent 

of the model and extended vertically throughout the model 

domain. These boundaries have an initial salt concentration of 

2.18 lbs/ft3 and equivalent freshwater heads that increase with 

depth on the basis of the vertical layering of the model. These 

boundaries are assumed to be seaward of the maximum extent 

of freshwater discharge and serve as a constant reservoir of 

saltwater required for the SEAWAT computations of the 

position and movement of the freshwater/saltwater interface 

(Guo and Langevin, 2002).

Streams and wetlands also were specified in layer 1 of the 

model as head-dependent flux boundaries because it is assumed 

that, under natural hydrologic conditions, these surface-water 

features receive ground-water discharge and that water is 

effectively removed from the ground-water-flow system once it 

discharges to a stream.

The altitudes of the streambeds and wetlands were 

estimated from 1:24,000 USGS topographic maps. The altitude 

estimate for coastal wetlands ranged from 2 ft to 4 ft above 

NGVD 29, this range is consistent with coastal marshes 

along Cape Cod Bay, such as Namskaket Marsh in Orleans 

(DeSimone and others, 1998). It was assumed for this 

analysis that the water levels in the streams and wetlands are 

approximately the same as the altitude of the streambeds and 

wetland surfaces because the water depths in the surface-water 

bodies in the study area typically are small (less than 2 ft).

The simulated hydraulic conductance was based on 

estimates from previous modeling investigations on Cape Cod 

(Guswa and LeBlanc, 1985; Barlow and Hess, 1993). The 

hydraulic conductance for the streambed and wetlands was 

calculated by the same equation (eq. 1.1) used to calculate the 

conductance of seabed sediments. The hydraulic conductivity 

was initially set at 30 ft/d; the width was 10 ft for the streams 

and 400 ft for the wetlands; and the length was 400 ft for both 

streams and wetlands.

These values were adjusted to improve the model 

calibration to the available field data. A previous analysis on 

western Cape Cod (Masterson and others, 1997) showed that 

large changes in estimated hydraulic conductance and 

streambed altitudes can affect the location and amount of 

ground-water discharge to surface-water bodies. Local-scale 

analyses of specific surface-water bodies may require more 

detailed hydrologic data collection than was possible for this 

regional analysis.

Initial Conditions

To simulate both freshwater and saltwater flow, an initial 

estimate of the altitude of the interface between freshwater and 

saltwater must be assumed. The better this estimate is, the less 

will be the simulation time required to achieve a final, stable 

solution of the interface (Guo and Langevin, 2002). The initial 

estimate of the position of interface in this analysis was from the 

coast to a uniform depth of 250 ft below NGVD 29. 

The simulation of flow and solute transport with 

SEAWAT requires transient conditions (Guo and Langevin, 

2002). Transient simulations were made from the initial 

concentration until a reasonable approximation of the altitude 

of the freshwater/saltwater interface was achieved and the 

model had reached a quasi-steady-state condition with respect 

to simulated hydrologic conditions. This solution was used as 

the pre-pumping condition for subsequent analysis of changing 

pumping and recharge conditions from 1907 to 2050.

The simulation of solute transport was made by an implicit 

finite-difference solution with advective transport only (Guo 

and Langevin, 2002) to calculate the position and movement of 

the transition zone between freshwater and saltwater. This 

solution was necessary because of the size of the model domain 

in this analysis, but also results in larger numerical dispersion, 

or smearing of the simulated transition between freshwater and 

saltwater. 

C is hydraulic conductance of the seabed (ft2/d);

K is vertical hydraulic conductivity of seabed deposits 

(ft/d);

W is width of the model cell containing the seabed (ft);

L is length of the model cell containing the seabed (ft); 

and

M is thickness of seabed deposits (ft).

C ( ) K ( ) W ( ) L

( ) M = ---------------------------
56 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

The resulting numerical dispersion from these simulations 

was similar in magnitude to the observed depth of the transition 

zone in the field data for non-pumping conditions (generally 

less than 40 ft thick). In the vicinity of simulated pumping 

wells, the numerical dispersion appeared to be much larger than 

the observed transition zone beneath the actual pumping wells 

(fig. 17). In these instances, the 50-percent salt-concentration 

contour was assumed to be a reasonable approximation of the 

interface between freshwater and saltwater.

Model Stresses

The hydrologic stresses simulated in the model consist 

of recharge and ground-water pumping. These stresses are 

simulated for transient conditions from an assumed quasisteady-state initial condition for the year 1906, the year before 

the start of continuous public-supply pumping on Lower Cape 

Cod. Simulations also were made for average annual recharge 

and pumping conditions, and for seasonally varying recharge 

and pumping conditions.

The length of the stress periods simulated in this transient 

model depended upon the analysis. A long initial period was 

simulated as part of the model-calibration process to solve for 

the position and movement of the freshwater/saltwater interface 

and to provide a best fit of model-calculated water levels and 

streamflow to observed data. The simulated elapsed time 

required for model calibration was not recorded because the 

iterative nature of the "trial-and-error" model-calibration 

process rendered this time meaningless. Once the model was 

calibrated, a 23-year stress period was simulated from 1907–29 

for average pumping conditions followed by annual stress 

periods from 1930 to 2002 to simulate actual pumping 

conditions over that period. 

Future scenarios were simulated for a 48-year-long stress 

period from 2002 to 2050 to assess the effects of pumping from 

proposed and existing pumping sites at proposed pumping 

rates. Because the flow model cannot simulate steady-state 

conditions, a long stress period of 1,500,000 days (more than 

4,000 years) was simulated to provide enough elapsed time to 

delineate the sources of water to public-supply wells, ponds, 

streams, and coastal areas under quasi-steady-state conditions 

by using the particle-tracking method detailed in Pollock 

(1994).

Forty-eight monthly stress periods were used to simulate 

the effects of in-season and off-season pumping and recharge. 

The 4-year period was chosen to provide enough time to 

minimize the transient effects of the initial condition and the 

monthly stress periods were used to minimize the potential 

effects of time discretization on the model-calculated results. 

Although there were 12 monthly stress periods per year, the 

pumping and recharge stresses were simulated at two different 

rates for in-season and off-season. The off-season pumping and 

recharge rates were simulated for seven stress periods per year 

(October through April) and the in-season stress rates were 

simulated for five stress periods per year (May through 

September). 

Recharge

Recharge rates specified in the numerical model were 

based on estimates of precipitation and wastewater disposal 

through domestic septic systems and sewage-treatment 

facilities. The recharge estimate of 24 in/yr was based on an 

assumption that about 45 percent of the annual precipitation of 

42 in/yr on Lower Cape Cod is lost to evaporation and plant 

transpiration and the remaining 55 percent enters the aquifer 

system as recharge, as was assumed in previous studies on 

western Cape Cod (Masterson and others, 1998). The average 

annual precipitation rate was obtained from the weather stations 

in Provincetown from 1948 to 1992 (National Oceanic and 

Atmospheric Administration, 2001), and South Truro from 

1980 to 2000 (National Atmospheric Deposition Program 

Station: MA-01).

The simulated recharge rate was adjusted downward 

for ponds, streams, and wetlands to account for increased 

evaporation from these surface-water bodies. Farnsworth and 

others (1982) estimate the average free-water-surface potential 

evaporation from surface-water bodies throughout Cape Cod to 

be about 28 in/yr. The combination of this evaporation rate with 

42 in/yr of precipitation results in a recharge rate of 14 in/yr 

from these surface-water bodies to the underlying aquifer.

An additional source of recharge is wastewater returned 

to the aquifer system from septic systems. Most of the 

communities of Lower Cape Cod obtain their drinking water 

from domestic wells and the resulting wastewater is returned 

to the aquifer system through on-site septic systems. For the 

purpose of this analysis, it was assumed that the water pumped 

from and returned to the same part of the aquifer system 

resulted in no effect on the flow system, and, therefore, recharge 

from septic wastewater was not simulated in the model. 

The drinking water for the residents in Provincetown and 

a small portion of North Truro is provided by a centralized 

municipal supply and the resulting wastewater is returned to the 

aquifer from on-site domestic septic systems. This wastewater 

return results in an increase in aquifer recharge in these areas. 

Eighty-five percent of the pumped water was recharged to the 

aquifer in proportion to the areas zoned for residential land use 

in those Traffic Analysis Zones (fig. 12) that are on municipal 
Development of Ground-Water Model 57

supply. The 15-percent reduction accounts for the consumptive 

use of water. In North Truro, municipal supply was provided to 

TAZ 8; about 0.1 Mgal/d of additional recharge was distributed 

over these areas for current conditions (2002). The remainder of 

the municipal supply from the North Truro area is exported to 

Provincetown in the Pilgrim lens, where it is the primary source 

of drinking water for its residents. Septic-system disposal in the 

Pilgrim flow lens recharges an additional 0.7 Mgal/d of water to 

the aquifer in TAZ 2 -6 (fig. 12) for current conditions. 

Provincetown is in the process of sewering the downtown area 

and plans to dispose treated wastewater from a centralized 

sewage-treatment facility to infiltration beds (fig. 12). Future 

pumping scenarios were based on the assumption that all of the 

water used in Provincetown (less the 15-percent consumptive 

use) is returned to the Pilgrim flow lens through the infiltration 

beds.

Simulations for Eastham and Wellfleet also were based on 

future pumping and recharge scenarios. In these towns the 

wastewater returned to the aquifer system was apportioned by 

pumping rates and distributed on the basis of areas of residential 

and commercial land use in TAZ 16–23 in Eastham and TAZ 

12–13 in Wellfleet (fig. 12). The total wastewater returned to 

the aquifer system in these areas was 85 percent of the water 

withdrawn at the proposed pumping rates for these towns. 

Changes in the annual average recharge from 

precipitation were made to simulate seasonal variability 

in evapotranspiration and the effect of reduced annual 

precipitation in response to a prolonged drought. The 

distribution of in-season and off-season recharge simulated in 

the model was based in part on the results of model calibration 

and on previous analyses conducted in the northeastern United 

States. One example is the Toms River area in central New 

Jersey where it was determined that about 4 in. of the 26 in. of 

annual recharge occurred from May to September (Nicholson 

and Watts, 1997). 

The average recharge in this study was divided into a 7-

month period of low evapotranspiration and 5-month period of 

high evapotranspiration. It was assumed that that 75 percent of 

the total annual recharge, or 18 in., occurs during the 7-month 

period from October to April. It was assumed that 25 percent 

of the total annual recharge, or 6 in., occurs during the 

remaining 5-month period from May to September when 

evapotranspiration is high. 

Wastewater and surface-water recharge rates also were 

varied seasonally. The wastewater disposal rates were highest 

for the in-season period when pumping rates were assumed to 

be highest. For the 7-month off-season period, wastewater 

disposal rates were lowest because pumping rates were lowest. 

All 14 in/yr of recharge into surface-water bodies was assumed 

to occur during the off-season period; no recharge was 

simulated during the in-season period. 

Pumping

The first public-water-supply system on Lower Cape Cod 

was installed in 1893 at the site known as the Auxillary well 

(fig. 12). This well was pumped at a rate of about 0.15 Mgal/d 

until it was discontinued in 1906 because of poor water quality 

in the Pilgrim flow lens (Craig Weigand, Provincetown Water 

Dept., written commun., 2001). Because this well had not been 

in operation for almost 100 years, it was not simulated in this 

investigation.

The earliest year of pumping simulated in this 

investigation is 1907, the first year that the the Knowles 

Crossing well field (also know as the Old North Truro well 

field) was in operation. This well field was developed in the 

Pamet flow cell to meet the demand for drinking water in 

Provincetown (Craig Weigand, Provincetown Water Dept., 

written commun., 2001). 

By 2002, the town of Provincetown had developed an 

additional water supply at South Hollow (fig. 12), which has 

been used since 1955, and had used the North Truro Air Force 

Base wells 4 and 5 seasonally from May to October since 1978. 

Provincetown also used the CCNS No. 4 site (fig. 12) from 

1978 through 1985 to meet the shortfall in supply caused by the 

gasoline spill near the South Hollow well field in 1978. 

Ground-water withdrawals were simulated for the publicsupply wells at their actual annual average pumping rates from 

1930 to 2002 with the exception of years 1940–43 and 1950, 

when no data was available (table 1-2). For these years, average 

annual pumping rates were estimated by taking the average of 

the pumping from the year before and the year after the missing 

data. 

From 1907 to 1929, the pumping from the Knowles 

Crossing well field was averaged and simulated as one pumping 

rate of 0.255 Mgal/d for this 23-year period. This was done to 

coincide with the model simulation of sea-level rise. For the 

purpose of this analysis, sea level prior to 1929 was assumed to 

be 0.0 ft NGVD 29 and to have risen 0.104 in/yr from 1929 to 

2002.

Future pumping scenarios were simulated for a total of six 

additional sites in Eastham, Wellfleet, and Truro (fig. 12). The 

pumping rates that were simulated at these sites were varied for 

each scenario to determine potential effects from pumping 

at these sites on surface-water bodies, the position of the 

freshwater/saltwater interface, and the source of water to other 

pumping wells.
58 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Table 1-2. Annual average pumping rates for Provincetown Water Department public-supply wells from 1907 to 2002, Truro, 

Massachusetts.

[All pumping rates are in Mgal/d. --, well not in operation]

Year

Pumping wells

Knowles

Crossing

South

Hollow NTAFB 4-5 CCNS Site 4

1907–29 †0.255 -- -- --

1930 .336 -- -- --

1931 .32 -- -- --

1932 .381 -- -- --

1933 .424 -- -- --

1934 .503 -- -- --

1935 .396 -- -- --

1936 .43 -- -- --

1937 .398 -- -- --

1938 .373 -- -- --

1939 .415 -- -- --

1940 *.410 -- -- --

1941 *.410 -- -- --

1942 *.410 -- -- --

1943 *.410 -- -- --

1944 .404 -- -- --

1945 .374 -- -- --

1946 .428 -- -- --

1947 .517 -- -- --

1948 .554 -- -- --

1949 .453 -- -- --

1950 *.465 -- -- --

1951 .476 -- -- --

1952 .456 -- -- --

1953 .399 -- -- --

1954 .488 -- -- --

1955 .228 0.364 -- --

1956 .213 .348 -- --

1957 .287 .373 -- --

1958 .235 .371 -- --

1959 .284 .384 -- --

1960 .131 .485 -- --

1961 .251 .425 -- --

1962 .172 .482 -- --

1963 .267 .466 -- --

1964 .229 .455 -- --

1965 .227 .43 -- --

1966 .285 .378 -- --

1967 0.416 0.28 -- --

1968 .312 .446 -- --

1969 .349 .476 -- --

1970 .448 .445 -- --

1971 .33 .507 -- --

1972 .311 .485 -- --

1973 .295 .556 -- --

1974 .44 .426 -- --

1975 .396 .479 -- --

1976 .503 .449 -- --

1977 .497 .5 -- --

1978 .489 .12 0.11 0.2

1979 .427 0 .167 .285

1980 .409 0 .138 .342

1981 .279 .196 .128 .263

1982 .196 .246 .008 .378

1983 .267 .224 .102 .267

1984 .271 .199 .174 .25

1985 .254 .42 .112 .052

1986 .093 .592 .131 --

1987 .083 .595 .183 --

1988 .062 .656 .109 --

1989 .1 .622 .107 --

1990 .084 .64 .108 --

1991 .093 .764 .08 --

1992 .151 .64 .1 --

1993 .236 .72 .112 --

1994 .266 .666 .123 --

1995 .329 .604 .176 --

1996 .262 .765 .121 --

1997 .238 .632 .164 --

1998 .268 .5 .163 --

1999 .277 .496 .128 --

2000 .248 .483 .132 --

2001 .15 .571 .11 --

2002** .2 .57 .165 --

Year

Pumping wells

Knowles

Crossing

South

Hollow NTAFB 4-5 CCNS Site 4

†Pumping rates for 23-year period averaged.

*Pumping rate estimated by averaging the 1 year before and 1 year after the year with missing pumping data.

**Estimated for 2002.
Development of Ground-Water Model 59

Agricultural, industrial, and small-capacity domestic 

pumpage was not simulated in the model because information 

for daily demand pumping less than 0.1 Mgal/d is not readily 

available, and because water pumped for these uses typically is 

returned to the aquifer system within the same model cells from 

which it is pumped. Therefore, it was assumed that pumping for 

these uses had a negligible effect on the aquifer system. Thus, 

the simulation of such small-scale pumping wells is beyond the 

scope of this regional analysis. 

Hydraulic Properties

The hydraulic properties required as input data for the 

ground-water modeling in this investigation are horizontal 

hydraulic conductivity, vertical hydraulic conductivity, 

porosity, specific yield, and storage coefficient. The 

determination of hydraulic properties was based largely on 

previous investigations throughout Cape Cod (Guswa and 

LeBlanc, 1985; LeBlanc and others, 1986; Barlow and Hess, 

1993; Masterson and Barlow, 1996; Masterson and others, 

1997) 

Hydraulic conductivities initially used in the numerical 

model were assigned on the basis of available aquifer test, 

lithologic, and geologic information. The Lower Cape Cod 

aquifer system has few deep wells; therefore, lithologic 

information necessary to determine hydraulic properties at 

depth is limited. Hydraulic properties of the aquifer material in 

areas with little or no lithologic information were estimated on 

the basis of the geologic processes that formed the glacial 

sediments of the ground-water-flow system.

The relation between geologic framework and hydraulic 

conductivities from a similar hydrogeologic setting on western 

Cape Cod (Masterson and others, 1997) was used for this 

analysis on Lower Cape Cod. The range in hydraulic 

conductivities in each of the model layers is shown in table 1-1. 

Hydraulic conductivity for model cells containing ponds was 

set equal to 50,000 ft/d. Streambed leakances were determined 

from estimates of the vertical conductance of the streambeds. 

The uniform porosity value of 0.3 used in the model is 

consistent with previous porosity estimates throughout Cape 

Cod (Garabedian and others, 1988; LeBlanc and others, 1991; 

Masterson and Barlow, 1996). The uniform specific-yield value 

of 0.25 is consistent with Moench (1994) and a uniform storagecoefficient value of 1x10-5 was based on Barlow and Hess 

(1993). In the cells representing ponds, the porosity, specific 

yield, and storage coefficient were set to values of 1.0 to 

account for the high storage capacity assumed for the ponds.

Model Calibration

The numerical model was calibrated to measured 

water levels, pond levels, streamflows, and the position 

of the freshwater/saltwater interface. Numerous water

level measurements were available from over 50 years 

of data available in the USGS online database (http:// 

waterdata.usgs.gov/nwis/gw). Streamflow data were limited to 

recent measurements made as part of the newly implemented 

National Park Service Long-Term Coastal Ecosystem 

Monitoring Program (McCobb and Weiskel, 2003). Data on the 

freshwater/saltwater interface were more difficult to obtain than 

water levels and streamflows because of the cost associated 

with drilling wells to the freshwater/saltwater interface. 

Available data from previous USGS studies (LeBlanc and 

others, 1986) and the town of Provincetown (Environmental 

Partners, Inc., 2002) were used for calibration as well as results 

from borehole geophysical logging and offshore marine 

resistivity surveys conducted as part of this investigation.

The water-level data used for model calibration included 

records from the USGS–Cape Cod Commission long-termmonitoring well network, and data from three synoptic waterlevel measurements on November 1975, June 2001, and May 

2002. The long-term monitoring well network consists of 13 

wells measured monthly or bi-monthly since 1976 (figs. 19 

and 1-2). Data from this network were not extensive enough to 

calibrate the model adequately. The mean and median water 

levels from these sites, however, were used to identify the dates 

of near-average water levels in more extensive synoptic 

measurements. 

Based on a comparison of the water levels measured in the 

long-term observation wells during more extensive synoptic 

measurements made in November 1975 and June 2001 with the 

mean and median water levels in the long-term observation 

wells, it was determined that these measurements were made 

when water levels were near-average conditions (see red 

symbols on fig. 1-6). The November 1975 synoptic water-level 

measurements were made in 46 wells; however, all but 13 

of these measurements were made in the Pamet flow lens 

(fig. 1-3). The June 2001 measurements were made in the 13 

long-term observation wells and in 19 observation wells from 

the newly implemented National Park Service monitoring 

program (fig. 1-2) (McCobb and Weiskel, 2003). In addition to 

the 32 water-level measurements, pond levels were measured in 

10 kettle-hole ponds and streamflows were measured at 8 

stream-gaging sites throughout the study area (fig. 1-4).
60 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Chequesset 

Neck

Pamet River

Herring R.

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

TSW-134

TSW-145

TSW-261 TSW-262

WNW-89

WNW-105

EGW-51

EGW-52

EGW-48 EGW-49

EGW-50

EGW-53

WNW-123

WNW-124

WNW-122

TSW-257

TSW-256

TSW-258

TSW 106

TSW-1

TSW 89

TSW-92

TSW-179

TSW-216

EGW-36

EGW-37

WNW-17

WNW-34

WNW-108

TSW-203

WNW-30

PZW-78

EXPLANATION

CAPE COD NATIONAL SEASHORE

PONDS, MARSHES, AND WETLANDS

RIVERS

LONG-TERM OBSERVATION WELL AND 

IDENTIFIER

NPS WELL AND IDENTIFIER

EGW-37

EGW-51

Figure 1-2. Locations of long-term observation wells and Cape Cod National Seashore coastal ecosystem wells, Lower Cape 

Cod, Massachusetts.
Development of Ground-Water Model 61

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Chequesset 

Neck

Pamet R.

Herring Riv er

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

EXPLANATION

46

1

3

4

5 6

7

8 9

10 11

12 13

14 16 17

18

19

15

20

21

22 23 24 25

26

27

28

2930 31

32

33

34

35

36

37

38

39

40

41

42

43 44

45

46

Figure 1-3. Locations of observation wells measured in November 1975, Lower Cape Cod, Massachusetts.
62 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

Chequesset 

Neck

Pamet River

Herring River

Snow Pond

70o15' 70o00'

42o

00'

41o

52'30"

0 5 MILES

0 5 KILOMETERS

Herring Herring

Pond

Ryder Pond

Herring

Pond

Gull

Pond

Great

Pond

Wellfleet

Duck

Pond

Pilgrim Lake

Long

Pond

Dyer

Pond

Great Pond-Truro

Little

Bennett

Pond

Little Pamet R

TSW-210–213

TSW-234–236

TSW-200, 224,226

TSW-219–222

WNW-80–83

WNW-50

EGW-41

WNW-117

Little Pamet River

at Corn Hill Road Pamet River

at Castle Road 

Herring River

at Bound

Brook Island

Road

Pole Dike Creek

at Bound

Brook Island

Herring River

at Herring Pond

Outlet

Herring River at

Old Kings Highway 

Fresh

Brook

at Route 6 

Hatches Creek

at West Road

C

C'

EXPLANATION

CAPE COD NATIONAL SEASHORE

PONDS, MARSHES, AND WETLANDS

RIVERS

LINE OF SECTION FOR MARINE RESISTIVITY 

SURVEY SHOWN ON FIGURE 1.8 

ZONE-OF-TRANSITION WELL AND IDENTIFIER

STREAM-GAGING SITES

POND-GAGING SITES

WNW-50

C C'

Figure 1-4. Locations of pond and stream-gaging sites, zone-of-transition monitoring wells, and line of marine resistivity 

measurements, Lower Cape Cod, Massachusetts.
Development of Ground-Water Model 63

As part of this investigation, a new network was 

established with the addition of 50 new observation wells and 

other available wells not included previously in the November 

1975 and June 2001 synoptic measurements to provide the most 

extensive water-level, pond-level, and streamflow-monitoring 

network to date. This network consists of 91 observation wells, 

10 ponds, and 8 stream-gaging sites and was measured over the 

period of May 22–25, 2002 (fig. 1-5) (Michaud and Cambareri, 

2003). These measurements, unlike the measurements made in 

November 1975 and June 2001, indicated conditions below the 

long-term average at most observation wells (fig. 1-6). 

Therefore, these measurements were used primarily for 

comparing model-calculated flow directions and the locations 

of the tops of the water-table mounds of the flow lenses in areas 

where data were previously unavailable (Scott Michaud, Cape 

Cod Commission, written commun., 2003). 

The streamflow data available for model calibration 

consisted of the monthly streamflow measurements made by 

the National Park Service from March 2000 to September 2000 

and from May 2001 to August 2001 (Evan Gwilliams, National 

Park Service, written commun., 2001); September 28, 2000 

(McCobb and Weiskel, 2003); August 1996 (Eichner and 

others, 1997), and measurements made as part of the synoptic 

water-level measurements in May 2002. 

Model calibration to the available streamflow data was 

complicated in that many of the streams are tidally affected and 

measurements of freshwater baseflow were difficult to obtain. 

For this investigation, the September 28, 2000, measurements 

(McCobb and Weiskel, 2003) and the measurements made by 

USGS personnel during the May 2002 synoptic event were used 

for model calibration. 

In the case of the Pamet River, which is affected by a tidecontrol gate that prevents the landward flow of saltwater at high 

tide, measurements made by Eichner and others (1997) were 

adjusted by a mass-balance method in order to account for the 

duration in which the tide-control gate was open for each tidal 

cycle. A freshwater discharge of about 3.0 ft3/s was calculated 

for the August 1996 measurements reported in Eichner and 

others (1997) (S.P. Garabedian, U.S. Geological Survey, 

written commun., 2002).

The third set of calibration data consisted of the positions 

of the freshwater/saltwater interface. These data consisted 

of the zone-of-transition data measured at eight locations 

throughout the study area from August 1973 to September 

1979 (LeBlanc and others, 1986) (fig. 1-4), geophysical 

measurements at eight deep monitoring wells at the three well 

fields in the Pamet flow lens from September 2000 to December 

2001, and the marine resistivity measurements made as part of 

this investigation in March 2001 (line of section shown on fig. 

1-4, section in fig. 1-8). 

The goal of the model-calibration process was to develop 

a model that provides results that compare reasonably well with 

available field data, so that simulations of future conditions can 

be made. Hydraulic conductivity was the parameter whose 

values were adjusted as part of this model-calibration process. 

Although simulated changes in aquifer recharge would have 

affected the flow system and the match between the model and 

observed field data, the recharge rate specified in this model 

was held constant throughout the model-calibration process. 

Determining the actual temporal and spatial changes in 

recharge was beyond the scope of this investigation.

An initial attempt was made to calibrate the flow model 

with a simulated constant sea-level altitude of 0.0 ft above 

NGVD 29. It was determined from these initial simulations that 

although the model provided a reasonable approximation of the 

position of the freshwater/saltwater interface relative to the 

available field data, the long-term water-level data consistently 

were lower than the water levels measured in the field. 

Changes in simulated hydraulic-conductivity values 

needed to increase model-calculated water levels resulted in 

an unacceptable match to data for the freshwater/saltwater 

interface positions beneath the pumping wells in the Pamet flow 

lens. Once the sea level rise of 0.104 in/yr (2.65 mm/yr) 

was incorporated into the simulations, the model-calculated 

water levels were a better match with the observed water levels 

(table 1-3) without a substantial change in the position of the 

freshwater/saltwater interface.

The model-calculated ground-water and pond levels are 

generally in close agreement with the average values reported 

on the long-term network wells, and with values measured in 

November 1975 and June 2001 (table 1-3). The means of the 

absolute error between the measured and model-calculated 

water levels for the average values reported for the long-term 

network wells, and for the measurements made in November 

1975 and in June 2001 were 0.50, 0.57, and 0.72 ft, 

respectively; these errors correspond to about 3 to 4 percent of 

the total relief of the water table of the Lower Cape Cod aquifer 

system. The median error between measured and modelcalculated water levels for these three datasets were -0.02, 0.17, 

and 0.07 ft (table 1-3). 

A comparison between the model-calculated and 

measured streamflow data is presented in table 1-4. 

Adjustments were not made to the model to closely match 

this streamflow data because of the limited number of 

measurements and complications with obtaining reasonable 

flow estimates; it was impossible to determine streamflows for 

average conditions. Therefore, these streamflow data were not 

emphasized in the model-calibration process, but were used as 

a general guide.

The comparison between model-calculated and measured 

positions of the freshwater/saltwater interface is complicated by 

the thickness of the transition between freshwater and saltwater 

in the observed data and the model-calculated spreading of the 

interface as a result of numerical dispersion. Therefore, it is 

difficult to quantify an absolute altitude of the interface in the 

field data and in the model-calculated results.
64 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Truro

Provincetown

Wellfleet

Wellfleet

Center

Eastham

Cape Cod Bay

Atlantic Ocean

Chequesset 

Neck

Pamet River

Herring River

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

0 5 MILES

0 5 KILOMETERS

70o15' 70o00'

42o

00'

41o

52'30"

EXPLANATION

8

6

6

6

8

3

2

5

4

6

3

2

4

6

2

4

6

6

5

5

2

8

7

7

16

10

14

12

Figure 1-5. Locations of observation wells, water-table altitude, and configuration, May 2002, Lower Cape Cod, Massachusetts.
Development of Ground-Water Model 65

WNW-30

2

4

6

8

10

1975 1980 1985 1990 1995 2000

YEAR

WNW-17

5

7

9

11

13

1975 1980 1985 1990 1995 2000

YEAR

EXPLANATION

TSW-89

2

4

6

8

1975 1980 1985 1990 1995 2000

TSW-216

2

4

6

8

1975 1980 1985 1990 1995 2000

PZW-78

2

4

6

8

1975 1980 1985 1990 1995 2000

EGW-36

10

12

14

16

18

1975 1980 1985 1990 1995 2000

WATER-LEVEL ALTITUDE, IN FEET ABOVE NGVD 29

WATER ALTITUDE

MEAN

MEDIAN

SYNOPTIC MEASUREMENTS 

FOR 11/75, 6/01, AND 5/02

Figure 1-6. Water-level altitudes at long-term observation wells EGW-36, PZW-78, TSW-216, TSW-89, WNW-17, and WNW-30, 

Lower Cape Cod, Massachusetts. Location of wells shown in figure 1-2.
66 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

Table 1-3. Measured water levels for selected observation wells and kettle-hole ponds in the modeled area in November 1975, and 

June 2001, and the median water-level altitudes of the long-term observation wells for measured period of record and model-calculated 

water-level altitudes for simulated current (2002) pumping and recharge conditions, Lower Cape Cod, Massachusetts.

[Map code, November 1975: Map code for figure 1-3. All values in feet above NGVD 29. NI, not included in network; --, not measured]

Wells

and

ponds

Model Map code, 

November

1975

Long-term 

measured

Long-term 

measured

minus model

November 

1975 

measured

November 

1975 

measured 

minus model

June 2001 

measured

June 2001 

measured 

minus model Row Column Layer

Pilgrim Flow Lens

PZW-71 34 19 1 1 -- -- 4.60 -0.14 -- --

PZW-74 38 55 3 4 -- -- 3.08 .17 -- --

PZW-77 36 44 3 3 -- -- 3.53 -.3 -- --

PZW-78 30 38 2 NI 4.85 -0.74 5.35 -.25 5.57 -0.02

Little Bennett 

Pond

30 28 1 NI -- -- -- -- 6.94 1.00

Pamet Flow Lens

TSW-1 76 76 5 7 2.51 0.2 2.69 0.38 2.57 0.26

TSW-87 98 87 3 20 -- -- 3.85 -.29 -- --

TSW-89 91 85 2 15 4.52 -.08 4.64 .04 4.83 .23

TSW-92 69 81 1 6 3.39 -.02 3.57 .16 3.55 .14

TSW-94 81 91 2 11 -- -- 4.68 .74 -- --

TSW-106 99 98 2 16 -- -- 4.86 1.26 4.11 .5

TSW-107 64 75 1 5 -- -- 2.19 .15 -- --

TSW-126 78 86 5 10 -- -- 4.18 -.15 -- --

TSW-132 79 94 1 9 -- -- 3.85 1.28 -- --

TSW-134 87 91 1 13 -- -- 4.91 .34 5.34 .77

TSW-136 84 85 4 12 -- -- 4.35 -.36 -- --

TSW-140 87 82 1 14 -- -- 4.12 -.2 -- --

TSW-142 92 78 2 18 -- -- 3.52 .75 -- --

TSW-145 96 82 2 NI -- -- -- -- 4.29 .07

TSW-153 98 92 1 17 -- -- 5.07 .59 -- --

TSW-157 100 80 4 19 -- -- 4.52 .74 -- --

TSW-162 110 77 1 21 -- -- 4.32 .11 -- --

TSW-165 118 73 1 36 -- -- 3.18 .51 -- --

TSW-166 120 73 1 37 -- -- 3.84 1 -- --

TSW-168 119 78 1 34 -- -- 4.8 .05 -- --

TSW-170 121 86 1 33 -- -- 5.84 -.41 -- --

TSW-173 122 103 2 22 -- -- 4.37 1.77 -- --

TSW-174 125 98 1 24 -- -- 5.42 .54 -- --

TSW-176 127 87 1 29 -- -- 5.44 -.48 -- --

TSW-177 125 78 1 35 -- -- 4.26 .03 -- --

TSW-179 135 85 1 32 4.62 1.04 4.51 .93 4.99 1.41

TSW-181 135 92 1 31 -- -- 4.72 .42 -- --

TSW-183 132 100 1 26 -- -- 4.77 .87 -- --

TSW-184 131 103 1 25 -- -- 4.45 1.92 -- --

TSW-186 138 103 1 27 -- -- 4.61 2.39 -- --

TSW-203 121 96 2 23 5.72 .08 5.5 -.14 6.27 .63

TSW-210 77 94 6 8 -- -- 3.36 .76 -- --

TSW-218 130 89 1 30 -- -- 5.14 -.24 -- --

TSW-258 113 90 1 NI -- -- -- -- 7.83 1.49
Development of Ground-Water Model 67

Chequesset Flow Lens

TSW-216 141 67 1 38 4.05 -0.55 3.64 -0.96 4.35 -0.25

TSW-219 140 104 6 28 -- -- 2.68 .24 -- --

TSW-256 160 89 1 NI -- -- -- -- 8.69 .16

TSW-257 156 75 1 NI -- -- -- -- 6.64 -.81

TSW-261 145 85 1 NI -- -- -- -- 7.68 .18

TSW-262 138 86 2 NI -- -- -- -- 4.21 -.07

WNW-30 184 77 5 40 6.53 -.56 6.5 -.59 7.06 -.03

WNW-34 176 85 3 39 7.98 .74 7.88 .64 8.5 1.24

WNW-78 173 48 3 41 -- -- -- -- 2.96 .12

WNW-89 206 79 2 NI -- -- -- -- 7.36 -2.12

WNW-105 176 85 1 NI -- -- -- -- 6.29 -.71

WNW-108 180 96 1 NI 7.52 1.36 -- -- 8.16 2

WNW-122 171 89 1 NI -- -- -- -- 6.64 -.12

WNW-123 198 85 2 NI -- -- -- -- 8.89 -.33

WNW-124 203 81 2 NI -- -- -- -- 9.01 -.68

Duck Pond 200 83 1 NI -- -- -- -- 7.97 -1.6

Dyer Pond 192 81 1 NI -- -- -- -- 9.27 .53

Great-Truro Pond 157 86 1 NI -- -- -- -- 9.06 .36

Great-Wellfleet 

Pond

194 87 1 NI -- -- -- -- 8.61 -.25

Gull Pond 179 90 1 NI -- -- -- -- 6.78 -.2

Herring Pond 172 90 1 NI -- -- -- -- 6.28 -.62

Long Pond 189 86 1 NI -- -- -- -- 8.76 .11

Ryder Pond 163 82 1 NI -- -- -- -- 8.09 .1

Snow Pond 162 85 1 NI -- -- -- -- 8.74 .37

Nauset Flow Lens

EGW-32 271 66 2 44 -- -- 12.4 0.17 -- --

EGW-36 269 64 2 43 13.22 -0.51 12.85 -.87 13.21 0.07

EGW-37 278 37 1 46 8.27 -.51 8.34 -.44 8.85 -.51

EGW-39 275 53 1 45 -- -- 13.85 -.09 -- --

EGW-48 277 65 3 NI -- -- -- -- 10.77 -.09

EGW-50 279 71 1 NI -- -- -- -- 6.46 -.04

EGW-51 282 45 3 NI -- -- -- -- 9.27 -.51

EGW-52 259 59 1 NI -- -- -- -- 17.36 1.06

EGW-53 275 75 1 NI -- -- -- -- 5.37 .84

WNW-17 234 77 3 42 8.59 .05 8.82 .28 8.78 .24

Mean absolute error 0.50 0.57 0.72

Median error -0.02 0.17 0.07

Table 1-3. Measured water levels for selected observation wells and kettle-hole ponds in the modeled area in November 1975, and 

June 2001, and the median water-level altitudes of the long-term observation wells for measured period of record and model-calculated 

water-level altitudes for simulated current (2002) pumping and recharge conditions, Lower Cape Cod, Massachusetts.—Continued

[Map code, November 1975: Map code for figure 1-3. All values in feet above NGVD 29. NI, not included in network; --, not measured]

Wells

and

ponds

Model Map code, 

November

1975

Long-term 

measured

Long-term 

measured

minus model

November 

1975 

measured

November 

1975 

measured 

minus model

June 2001 

measured

June 2001 

measured 

minus model Row Column Layer
68 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

1Calculated to be 3.00 cubic feet per second from August 1997 

measurements.

For the purpose of this analysis, the 50-percent salt 

concentration was assumed to be a reasonable approximation of 

the freshwater/saltwater interface. A comparison of the modelcalculated interface and the observed interface position is 

presented in table 1-5 for the eight zone-of-transition wells. 

Profiles of the model-calculated salt concentrations and the 

electromagnetic-conductivity geophysical logs measured in the 

eight deep monitoring wells at the three well fields in the Pamet 

flow lens are shown in figures 17 and 1-7.

Additional data used for model calibration to the position 

of the freshwater/saltwater interface was from the offshore 

marine resistivity survey conducted by the USGS Branch of 

Geophysical Applications and Support in March 2001 (Eric 

White, U.S. Geological Survey, written commun., 2001). A 

series of transects was made in Cape Cod Bay offshore of the 

Nauset flow lens to confirm the model-calculated subsurface 

seaward discharge of freshwater into Cape Cod Bay. 

An example of the comparison between the modelcalculated offshore extent of freshwater seaward discharge into 

Cape Cod Bay with marine resistivity results from that same 

area is shown in figure 1-8 and shows that the two are in close 

agreement. The model calculated that freshwater would 

discharge offshore into Cape Cod Bay because of the effect the 

thick deposits of low permeability silts and clay underneath the 

Nauset flow lens has on ground-water flow in the aquifer. The 

marine resistivity technique proved to be a useful means of 

determining the offshore seaward extent of freshwater 

discharge and provided the field data necessary to confirm the 

model results; these data could not have been readily obtained 

otherwise. Additional information on the marine resistivity 

technique for determining the offshore discharge of freshwater 

can be found in Manheim and others (in press; 2002). 

Table 1-4. Measured discharge for selected streams in the 

modeled area in September 2000 and May 2002, and modelcalculated streamflows for simulated current (2002) pumping and 

recharge conditions, Lower Cape Cod, Massachusetts.

[Locations shown on fig. 1-4. All values are in cubic feet per second. --, not 

available]

Stream September

2000 May 2002 Modelcalculated

Little Pamet River 1.5 1.2 0.61

Herring River at pond .22 1.54 .11

Herring River at Kings 

Highway

.22 1.5 .11

Pole Dike Creek -- 2.09 1.92

Herring River at Bound 

Brook Road

4.06 5.43 6.67

Hatches Creek .09 .22 .52

Fresh Brook .79 .7 .57

Pamet River1 -- -- 2.98

Table 1-5. Measured altitude of the transition between 

freshwater and saltwater at selected zone-of-transition wells from 

LeBlanc and others (1986) and the model-calculated 

freshwater/saltwater interface for current (2002) pumping and 

recharge conditions, Lower Cape Cod, Massachusetts.

[Well locations on figure 1-4. Altitude with respect to NGVD 29, in feet. 

Model 50 percent: Model-calculated 50 percent salt concentration. ZOT, Zone 

of transition; --, not available]

Wells ZOT top ZOT bottom Model 50 

percent 

TSW-210–213 -70 -- -95

TSW-234–236 -110 -160 -185

TSW-200, 224, 

226

-170 -250 -245

TSW-219–222 -40 -60 -75

WNW-80–83 -40 -70 -62

WNW-50 -60 -70 -80

WNW-117 -55 -75 -115
Development of Ground-Water Model 69

50%

25%

75%

50%

25%

25%

75%

75%

50%

EXPLANATION

EM CONDUCTIVITY, IN MILLISIEMENS PER METER

ALTITUDE, IN FEET BELOW NGVD 29

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

KC-1 KC-3 KC-2

0 100 200 300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5

0 100 200 300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5

0 100 200 300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5

0

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0 0

A.

SALT CONCENTRATION, IN PERCENT

MODEL-CALCULATED SALT CONCENTRATION

EM CONDUCTIVITY, MEASURED SEPTEMBER 2000

25%

Figure 1-7. Profiles of electromagnetic (EM) geophysical logs at A, Knowles Crossing; and B, North Truro Air Force Base 

wells 4 and 5 measured in September 2000 and model-calculated changes in salt concentration with depth, Truro, 

Massachusetts.
70 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

EXPLANATION

EM CONDUCTIVITY, 

MEASURED SEPTEMBER 

2000

MODEL-CALCULATED SALT 

CONCENTRATION

SALT CONCENTRATION, IN 

PERCENT

25%

25%

75%

75%

50%

50%

25%

EM CONDUCTIVITY, IN MILLISIEMENS PER METER

ALTITUDE, IN FEET BELOW NGVD 29

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

SALT CONCENTRATION, IN

POUNDS PER CUBIC FOOT

NTAFB-4 NTAFB-5

0 100 200 300 400 500 600 700 800

-250

-225

-200

-175

-150

-125

-100

-75

-50

-25

0

0.0 0.5 1.0 1.5 2.0 2.5

0 100 200 300 400 500 600 700 800

-250

-225

-200

-175

-150

-125

-100

-75

-50

-25

0

0.0 0.5 1.0 1.5 2.0 2.5

B.

Figure 1-7—Continued. Profiles of electromagnetic (EM) geophysical logs at A, Knowles Crossing; and B, North Truro Air 

Force Base wells 4 and 5 measured in September 2000 and model-calculated changes in salt concentration with depth, 

Truro, Massachusetts.
Development of Ground-Water Model 71

SALTWATER SALTWATER

TRANSITION ZONE TRANSITION ZONE

FRESHWATER FRESHWATER

0 2,000 1,000

DISTANCE FROM SHORE, IN FEET

0

80

20

40

60

SALTWATER

TRANSITION ZONE

FRESHWATER

West East

West East

0

 50

 100

 150

200

250

 300

 350

400

450

500

DEPTH BELOW NGVD 29, IN FEET

DEPTH BELOW

NGVD 29, IN FEET

CAPE COD BAY ONSHORE-EASTHAM

VERTICAL SCALE GREATLY EXAGGERATED

Colors from the resistivity profile and model section

are not correlated exactly.

0 2,000 FEET

0 500 METERS 

EXPLANATION

FRESHWATER ZONE

TRANSITION ZONE

SALTWATER ZONE

c c'

A.

B.

Figure 1-8. Cross sections showing A, the marine streaming resistivity profile; and B, the model-calculated boundary 

between freshwater and saltwater, Eastham, Massachusetts. Section line C-C′ on figure 1-4.
72 Simulated Interaction Between Freshwater and Saltwater, Lower Cape Cod, MA

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